Homebrew L-match

Having not posted in a while I figured I would show the innards of my homebrew l-match.

There is some information written on the chassis. The picture below is upside down to be corrected later... or not...

Enjoy!

RTL SDR up-converter for HF use

I picked up a RTL SDR from amazon a while back for around 20 bucks. It works fine on VHF/UHF. I wanted to try it on HF. Typically these inexpensive devices can not go below about 25MHz. In order to listen in on some MF and HF stuff I decided to throw together a quick and dirty up converter.

With up-conversion I am basically shifting the lower HF frequencies into a range that the SDR dongle can see. In this case with a 7MHz base frequency into the up-converter, the SDR will see it as 32MHz.

https://www.amazon.com/RTL-SDR-Blog-RTL2832U-Software-Telescopic/dp/B011HVUEME/ref=sr_1_4?ie=UTF8&qid=1504202463&sr=8-4&keywords=rtl+sdr

Using what I had laying around, I found a 25MHz crystal so I made a crystal oscillator and buffer going into a double balanced mixer. This is all built into an Altoids container with a pigtail of RG-174 to go to the SDR reciever and a BNC for the antenna connection. The setup runs on a 9 volt battery.

Some images follow of the up converter below as well as a couple test videos of reception.

Transformers wound on binocular cores from science and surplus. -AC9LF

Oscillator and buffer circuit with 25Mhz crystal. -AC9LF

Double balanced mixer added with 9V connector. -AC9LF

I may change the oscillator to 40MHz because of harmonic mixing. For example at 50MHz which is double the 25Mhz crystal frequency the up conversion starts all over again. It would not work well at 10 meters.

I put a simple low pass RC filter at antenna end with the cut off at about 30Mhz the values were 22ohms and 220pF respectively. I might change this to a better LC type filter later. Initial testing seemed promising with a small ~2 foot antenna hooked up as you can see in the short video below.

This first video is of a couple AM stations on the up converter. 

This next video is of some SSB on 40 meters.

I may edit this with a schematic later. I don't have anything scratched down right now.

Crude up-converter doing its thing. -AC9LF

I used Cubic SDR and Ubuntu Mate for the PC side of things.

Edit: 10/09/2017 schematic below.

https://drive.google.com/file/d/0B-GeQSy2vlYfSjMxVUM4ZWYzUTQ/view?usp=sharing


SSB reception with magnetic loop antenna attached.

Yet another tape measure yagi

I built this a while back.

This is my build of a tape measure yagi antenna centered around 146Mhz. I used a harbor freight tape measure as the sacrifice for the antenna elements. It is all based on this really well done drawing by NT1K.

Fox hunting, also known as radio direction finding was my intention with this antenna.

I made heavy use of duct tape both to hold the elements and to cover the ends to prevent injury. I did use steel hose clamps for the driven element because this made it easier to adjust. The hose clamps are not as cheap as duct tape so duct tape it is. I also didn't get another cross connector for the end since the T was slightly cheaper. It works pretty well and I was able to find several VHF noise sources in my house with it.

I just used RG-174 with an SMA connector on the end for use with a handheld transceiver.

On the back side you can see the hair pin matching wire soldered to the tape measure driven element.

Top -AC9LF

Bottom with matching hairpin -AC9LF

Drawings with associated location of posting:

http://nt1k.com/blog/2012/vhf-3el-tape-measure-yagi/

http://nt1k.com/blog/2012/vhf-3el-tape-measure-yagi/

http://nt1k.com/blog/2012/vhf-3el-tape-measure-yagi/

It's currently folded up waiting for a fox hunt adventure. I will also create a step attenuator for it at some point.

Simple homebrew programming cable for Kenwood radios (TM-271)

I went to the South Milwaukee swap fest this weekend on the look out for another 2 meter rig I could possibly use in my car. I found a Kenwood TM-271 for a good price in mint condition. My ADI rig does not have the ability that I am aware of to program it with a PC. This Kenwood does. I did not want to purchase a programming cable for it as I am aware that most of these radios use some form of TTL serial port for its programming communications method. I think this should work on other Kenwood mobile rigs with the same mic connection scheme.

The Kenwood uses a standard RJ-45 jack for the Mic connection and uses this same jack for programming. You can use a standard Ethernet patch cable for this. Look below to verify your sacrificial Ethernet cable has the colors in normal patch cable order as shown below.

Ethernet patch cable pinout

Looking through the schematic of the service manual for the Kenwood I verified the proper wiring for the transmit pin, the receive pin and ground. I did this by just measuring the two ground pins to the chassis with no power then referenced these pins to the "8 volt" power out pin. If you follow the schematic you find that the HOOK and PTT are also the RX/TX respectively from the Kenwood radio.

For connecting the the Kenwood I found that the wire colors with regard to the standard Ethernet patch cable color code were as follows:

Ethernet Orange: RX from Kenwood (Kenwood Pin 2 HOOK/RXD)
Ethernet White/Blue: TX to Kenwood (Kenwood Pin 5 PTT)
Ethernet Green: GND (Kenwood Pin 6 GND)

Kenwood service manual snippet.

I used an FTDI breakout board which is a USB to serial TTL converter. I have this available for talking to microcontrollers I play around with. I left the levels set to 3.3 volts per the jumper on my breakout board I recommend doing the same. 

How this works:

• All you have to do is cut an Ethernet cable to the length you want (within reason, a couple feet should be fine). 
• Strip the orange, white/blue and green wires. 
• (Note: white/blue is one wire, white with a blue strip)
• (Optionally) Tin these wire ends with solder to make it easier to insert into a bread board. 
• (Optionally) You should also trim/tape off the wires not used to prevent them from shorting or causing issues.
• Using an FTDI breakout board connect as follows
• Orange to TX on FTDI
• White/Blue to RX on FTDI
• Green to GND on FTDI
• Connect your radio to a power source and plug in the USB cable for the FTDI.
• Plug the RJ-45 into your radio open Chirp or your favorite programming tool and away you go!
• I recommend to have a dummy load connected when doing this. The PTT is used as one of the programming pins. If something strange happens the radio could potentially go into transmit. Better to be safe then end up with a dead final.

This worked for me using Chirp software and is very easy. I can now easily change my frequency memory list when ever I want to go on trips.

Bread board with FTDI and cable wires. -AC9LF

Here is a link to a breakout board that should be usable for this type of thing:

https://www.amazon.com/Qunqi-FT232RL-Serial-Adapter-Arduino/dp/B014Y1IMNM/ref=sr_1_4?ie=UTF8&qid=1499692518&sr=8-4&keywords=ftdi&refinements=p_72%3A2661618011

If you are attempting to do this or anything presented on this blog I am not responsible if anything bad results. Sometimes being cheap can be expensive if you end up turning your fancy radio into a brick. You are responsible for your own actions.

Good luck!

A Beach 40 DSB Rig built by AC9LF

A while back I decided to build a transceiver based on the "Beach 40" double side band rig by Peter Parker (VK3YE). This would be my first transceiver home brew project. The "Beach 40" is a 40 meter double side band direct conversion transceiver with a (originally) minimum of parts. It has some frequency agility and should put out a few watts.

I wanted to used mostly parts I already had on hand. I started off with the VXO which is a ceramic resonator variable oscillator. As you can see below I built it into an Altoids tin. These seem to be ideal for RF, it is shielded and actually very solderable. The chassis of the can was used as the ground plane, ugly type construction was used. Leads were kept short as best I could.

Start of the oscillator for the DSB transceiver. -AC9LF

I decided on using varactor type tuning instead of going with a variable capacitor. I did have some proper varactor diodes but I did not get much tuning range out of them. I think I ended up using a couple of parallel 1N4001 type diodes as the varactors with good results. From the image below you can see the varactor diodes, a paralleled ceramic resonator and a small variable capacitor. Paralleling two ceramic resonators extended the tuning range of the oscillator. The variable capacitor was used to set the area of the band I wanted to tune in. In this case around 7.200MHz. I used a pot from a dead function generator for the tuning pot. I also used a voltage regulator for a stable tuning voltage across the tuning diodes.

Testing oscillator -AC9LF

Oscillator output with associated FFT. -AC9LF

I added a balanced mixer in the Altoids tin, winding the single transformer on a binocular core ferrite that I picked up from Science and Surplus for around 40 cents. The blue potentiometer is used to null out the carrier leaving only the upper and lower sideband signals.

Mixer added. -AC9LF

Output of mixer with an audio input test signal. -AC9LF

I built the rest of the rig on a piece of copper clad board starting with the low pass filter. I used toroids pulled from a dead PC for the inductors. I wound and measured the inductance before soldering them in and securing them with hot glue.

Low Pass Filter -AC9LF

I built the transmitter section starting with the mic amplifier. I then proceeded RF amplifier stage by RF amplifier stage. I made use of the binocular core ferrite as needed. I used the microswitch below to enable the transmitter for taking measurements on the oscilloscope. The center relay was put on dead bug style to switch between the transmit and receive circuits.

Beginning of the transmitter. -AC9LF

I built the audio output amplifier with an LM386, a jumper is available to switch between a fixed 20x gain or 200x gain. The screw terminal block goes to an 8 ohm speaker. The input the the LM386 has a potentiometer for volume control or "AF gain". I did use some perf board for the IC and construction of the audio amplifier. I have a heat sink on both the final transistor and the driving transistor. The driving transistor seemed to dissipate more heat then the actual final. The RF choke on the final transistor is just another scrap toroid that I wound a few turns of wire around. The output varies a bit with the number of turns on this core. For initial testing the mic and BNC were soldered directly to the PCB to be transferred the a makeshift case later.

Running out of space! -AC9LF

During some initial testing the rig seems to put out about 1 watt. I will need to see if I can squeeze any more out of it. There also seems to be a little bit of self oscillation initially at around 1MHz if I recall.

DSB RF output into 50 ohm dummy load. -AC9LF

I stuffed everything into a scrap enclosure I found. It seemed to be just big enough to fit. I still have to play around with the hardware a little more to get some more output power on it and see what it can do on the air. In the mean time I have added the schematic I based this on below. It was found on the soldersmoke blog. I did add a small filter to the front end receiver to try and remove some AM broadcast overload I was getting. I should be able to post more about this later. It has been sitting on the shelf for several months now collecting dust. I will get back to it soon...

http://soldersmoke.blogspot.com/2012/11/schematic-of-vk3ye-dsb-on-beach-rig.html

http://soldersmoke.blogspot.com/2012/11/schematic-of-vk3ye-dsb-on-beach-rig.html

Using an Xbox power supply for other things

I was walking along one day and just happened to glance in the garbage, lo and behold I found a 16.5 Amp 12 volt power supply! I have converted the proprietary connector of the power supply to something more useful to me.

I have started using molex connectors from PC power supplies for my 12V power connection pigtails on my home systems. These are pretty easy to find. The male end is a little more difficult. I spliced in this connector using the 2 inner pins of the molex connector for ground and the outer ones for +12V.

http://photobucket.com/gallery/http://s293.photobucket.com/user/B-e-t-a/media/Fan%20write-up/1_PSU_molex_connector.jpg.html

http://forums.reprap.org/read.php?219,191038

I attached a switch to the blue and red wires on the power supply to turn it on and put it back into standby. The measured standby power in is about 1 watt, not bad but not great either.

Cut off the old connector -AC9LF

New fancy switch to bring power supply out of standby mode. -AC9LF

I tried plugging in a standard IEC computer power cord into the power supply and found that there was a tab in the way. It looks like this is a 120Vac input only power supply and it may be keyed to prevent use on 240Vac systems. I didn't have a worry about that as I only have 120Vac outlets around. I snipped off the plastic and was good to go.

Defeated tab for regular IEC use. -AC9LF

I connected this power supply to my 2 meter VHF radio and was pleasantly surprised to find that the power supply did not produce any noticeable RF noise on the 2 meter band. I have yet to test it with my HF radio running, but so far it seems to be pretty clean.

New connector plugged into VHF radio. -AC9LF

QRP magnetic loop antenna for 40 meters

I know I will have to start doing a bit more with circuits on this blog of mine. Lately I have been focusing more on antennas. This post is no exception. I have been sitting on this magnetic loop antenna for some time. It has been near complete in my basement since early winter. The reason I have not completed it is that final tuning and testing needed to take place outside. Winter makes it cold and miserable out and there is far too much metal in my basement for it to couple to and throw everything off.

This weekend was great, yesterday was sunny and around 80F. I did try taking this thing outside once before but I made the loop too large to fit through the stair well. The original circumference of the main element was about 18 feet with original testing and experimenting done in the basement during the winter. I had to cut this down and make it closer to 16 feet.

I used this site to calculate the antenna characteristics: http://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx

Magnetic loop leaning on tree - AC9LF

I used a 20 foot roll of 1/4" copper tube from the local Lowe's store which cost about 10 bucks. The antenna isn't very efficient per the calculation from the above website at 5 watts:

Note that these numbers are a bit off from what I measured:

Antenna efficiency: 23% (-6.4 dB below 100%)
Antenna bandwidth: 15.1 kHz
Tuning Capacitance: 157 pF

Capacitor voltage: 580 volts RMS
Resonant circulating current: 4.11 A
Radiation resistance: 0.034 ohms
Loss Resistance: 0.114 ohms
Inductance: 3.12 microhenrys
Inductive Reactance: 141 ohms
Quality Factor (Q): 477
Distributed capacity: 13 pF

I measured the main loop with my LC meter which turned out to be about 4.6uH, The variable capacitor I used is of the butterfly type. This allows lower loss because there is no mechanical wiper connection. It also allows higher voltages across the capacitor. The trade off is that the capacitance is lower and butterfly capacitors go from fully meshed to fully unmeshed with a 90 degree change in rotation where as standard variable capacitors are 180 degrees. For my purpose this turned out to be perfect as I was only interested in covering the 40 meter band which is 300khz wide. Measuring the capacitor it is roughly 5pF to 20pF. 

Close up of capacitor and connections -AC9LF

To resonate the main loop at 7Mhz with 4.6uH of inductance I needed roughly 113pF of capacitance. With only the small capacitance of the variable butterfly capacitor I needed to add some capacitance in parallel. I chose to go with RG58 a/u as my extra capacitance. This coax uses solid polyethylene insulation which increases its maximum voltage rating over foam insulation. According to a datasheet the max voltage is about 1,400V RMS. This is well above the 580V RMS calculated at 5 watts. Capacitance per foot is around 30pF so I cut a length of slightly over 4 feet to bring me a bit above the 100pF needed for resonance. 

I trimmed the coax away slowly which shifted the frequency up to 7Mhz with the plates of the capacitor fully meshed. I used the receiver of the VX-7R to follow the frequency with the highest noise level while pruning the coax. The frequency with the highest noise level is the frequency of resonance. This was confirmed with my Dip Meter. Once satisfied I switched to my BITX 40 to perform final transmitter SWR testing.

Here is the data sheet for Belden RG-58 a/u: http://www.belden.com/techdatas/english/8259.pdf

I used something a bit different but should be pretty similar in characteristics.

Coax used as parallel capacitor - AC9LF

This combination gave me a calculated tuning range of a bit more then 300Khz which as stated before fits the whole 40 meter band. It also allows for easier tuning as having a wider tuning range makes it harder to hit the resonant "sweet spot" at about 50 ohms for the transceiver to be happy. I used a hard plastic straw on the shaft of the capacitor to decrease coupling between me and the antenna when tuning it.

The frame is made out of remnants from a wood trellis that I scrapped. The inner driven loop is made out of the same RG58 a/u coax cable with one end of the shield connected to the center conductor. The center conductor on the opposite end of the driven loop is connected to the center conductor of RG-174 which has a SMA connector on it. The shield of the RG-174 goes to the Shield/Center conductor side. Connecting the outside shield to the inner conductor on the driven loop helps to reduce electric field noise pick up. The circumference of the driven loop is 1/5 of the outer loop. 

Using the setup below I was able to get SWR close to 1:1 on any frequency of interest on the 40 meter band. Even though fully meshed to fully unmeshed of the capacitor only covers a little more then 300Khz hitting the "sweet spot" was still a little tricky. I plan on putting a stepper motor on this in the future for remote tuning. This will also allow micro stepping for more accurate and easier tuning of the capacitor.

QRP setup -AC9LF

Even though the XYL says it isn't, this really is a small antenna seeing as a dipole on 40 meters would be around 60 feet long.

I still have yet to make a 40 meter contact with the BITX40. My operating skills must not be up to par for QRP.

Bicycle mobile VHF antenna an AC9LF special

I don't have a very long commute to work. Usually it only takes me around 6 minutes to get to any place I need to go. I live very close to grocery stores, hardware stores and my place of work. That said I really don't have any road time to justify putting a mobile radio setup in my car at the moment. I still wanted to do some kind of mobile operating and my focus shifted to be bike.

One of my favorite things to do in the summer is ride my bicycle. Sometimes the rides are a little long and if I don't have a riding partner can be a bit dull after a while. I considered combining amateur radio and bicycling. It's been done, and some setups are very impressive and eye catching. Not something I am interested in. I would like my setup to be fairly low profile, yet effectively be able to hit the local repeaters around the city running low power on a small handheld transceiver.

Reading on the various ways others have mounted antennas on bikes I found two main ones. Either mounted on some kind of bracket on the back of the bike or attached to the 6 foot pole of a fiberglass safety flag.

The safety flag setup is usually done with a wire J-Pole twin lead design such as this one here:
http://www.bikexprt.com/bicycle/antennae.htm

I thought I could go a little lower in the profile as this antenna is pretty high. I may still have a go at it sometime but for now I decided on trying to use a quarter wave antenna to cover the 2 meter band. I wanted to also try using the bike frame as the counter poise to this antenna. I read that the frame typically does not make for a good counterpoise but I wanted to try it anyway. There seems to be enough material on the bike to act as one on 2 meters and up.

I was looking into ways to mount this antenna. Some people constructed their own brackets but most just attached the antenna to a rear rack mount. Usually the antenna was attached with some kind of connector like an NMO or SO-239 mount which was mounted through the rear rack mount on the bike. I looked at these as well as antennas on Amazon. Most of what I found didn't appear to be tunable. Meaning that you could not pull out the antenna element and trim it to resonance for the frequency range you want to use it on. This may be necessary, especially when attached to an odd structure like a bicycle. It is by no means a car roof. They also seemed a little pricey for just a stainless steel rod of a certain length to be screwed on to a mount.

I did spend a little bit of money on a rear rack mount for my bicycle from REI, (I know right, I'm supposed to be cheap!) Well the thing cost me a little over $1.00 after my 20% discount and the dividend from what the XYL must have spent some money on the previous year or two. I mounted this to the bike and not having any commercial antenna to put on it I built one with what I had laying around.

Playing the role of Macgyver I pieced together an antenna from a small block of scrap wood to be used as the center insulator, a scrap section of copper tubing, a length of scrap coax with an SMA connector from an old cell phone antenna, the left over BNC cap from the bitx 40 kit, a bolt, a washer, a nut, zip ties and finally a generous portion of hot glue.

I drilled a hole about the same diameter of the copper tube in the wood, almost but not all the way through. I drilled a second hole about halfway through the wood to fit the bolt I had. The head of the bolt would rest at this half way point. A smaller hole was drilled for the bolt to go through the wood and through the rear reflector attachment hole in the bike rack. The bottom side of the rack was sanded down to remove the paint to make electrical contact between the bare aluminum rack plate and the bolt.

Sanded aluminum area - AC9LF

The copper pipe was inserted into the wood and the center conductor of the RG-174 coax was soldered to it at the base protruding from the wood. The shield was clamped to the wood with the screw head and makes electrical contact with the frame through the nut at the bottom. The coax was wrapped around the bar on the rack about 10 times to make an attempt at a choke. A small piece of ferrite was clamped on at the feed point as well. The effectiveness is currently unknown. The coax was secured to the bike frame up to the handle bars with zip ties and the feed point covered in hot glue. The glue was built up a bit at the base of the antenna to help secure it structurally to the piece of wood. There is no other mechanism securing the pipe other then friction and glue.

HOT GLUE!!!! - AC9LF

Choke  - AC9LF

Tuning a copper pipe to resonance takes some time... I cut off 1/4" sections at a time with a pipe cutter. The starting length was about 19.25". The end length is around 17". I was able to get an SWR of less then 1.5:1 across the 2 meter band with some variation when I mount the bike. I also appear to get low SWR on a usable part of the 70cm band. The plastic cap is to help prevent possible impalement if I were to somehow fall on the antenna portion. Chances are, this wont happen. If it does it is more likely the antenna will bend or break off instead.

Finished junky antenna - AC9LF

I'm using an old camera bag secured to the handlebars with zipties to hold the HT.

I still have to take it for a test spin to see if it falls apart. I have some extra zip ties to secure it to the frame if it does fall off during cycling. Future improvements may be to add a connector to make it easier to remove when needed. I'm excited to give this a go.

Cheap, simple, and effective urban dweller tv antenna

Most over the air television antennas (indoor) that you would buy at stores are usually full of gimmickry. They tend to say that they are the smallest and have the highest gain and usually include a pre amplifier of some kind. In my experience pre amplifiers usually cause more issues than they help. All of these antennas are compromise antennas, and usually none are very effective.

Some of them are just flat plastic boxes with a small coax cable that plugs into your television. Others have a telescoping antenna element, just one which would be a monopole that has no counter poise. All very poor.

Granted there will be no one simple antenna that will efficiently cover the broad range of frequencies that encompass the over the air television spectrum. Log periodic yagi type antennas are the exception but of course more complex. What is shown here is also a compromise antenna in that it is not tuned for any particular frequency. Nevertheless, it is very simple and fairly effective in an urban environment where television broadcast signals are fairly strong. If you don't live near broad cast stations this is not for you, you need height and gain.

I stumbled upon this idea, it is not my own. Simply, you take a metal coat hanger cut the hook off, shape it a bit so that the distance between the top and lower half are even. You then take a 300 ohm to 75 ohm transformer available at most local stores that carry television antennas or television accessories. The 300 ohm twin lead end gets connected to the cut portion of the antenna coat hanger. This assembly gets mounted to an insulator of some kind in my case I used a piece of scrap wood which also allows me to stand it up and let it lean against a wall near the television with the element horizontal. That makes for the complete antenna, very simple. All you have to do now is connect it to your TV with some 75 ohm coax.

This antenna is called a folded dipole which typically has a feed point impedance of around 300 ohms at its resonant frequency. At other frequencies this will vary but that impedance transformer helps to mitigate the possible impedance swings vs frequency at the antenna by its ratio of 4:1.

Brief practical non-scientific testing has shown this to be a fairly decent performer all local stations come in with around middle level signal strength as shown on the television used for testing. The television under test is located in a low area of the city. Signal strength typically in this area is usually poor with indoor antennas. 

This antenna is bi-directional in that you use it like a normal dipole and have it pointed in the direction of the station. In this case most broadcast stations are located in about the same direction. This antenna can also be expanded on later by adding a reflector this will create a nice gain directional antenna.

Image below, have fun!

Using 75 ohm coax with a 50 ohm vhf transceiver

RG6 is cheap, really cheap ( 🐦 ) compared to the lowest cost RG58. The losses according to data sheets are lower then the more expensive RG58 and can even compete with the very expensive RG8 stuff.

http://rfelektronik.se/manuals/Datasheets/Coaxial_Cable_Attenuation_Chart.pdf

For around 30 dollars from the local home improvement store you can get a 500 foot spool of RG6 where as RG58 can cost around 350 bucks for the same length online.

Using RG58 at VHF or greater frequencies is not recommended. The loss can be severe especially with longer runs. Being a very frugal guy I got to wondering, why isn't RG6 used instead? Why are transceivers designed with a 50ohm output? What happens if I just connect RG6 to my radio with a properly tuned antenna on the other end? How can I make this work?

Let's find out...

After some online research I found out that 50 ohm coax was chosen as a compromise between signal loss and power handling ability. You will notice that rg6 is used everywhere for cable, TV, satellite etc. These are all receivers or sometimes low power transmitters in the case of cable Internet. The 75 ohm coax is the impedance of cable with the lowest loss but suffers a bit on its power and voltage handling capability.

http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm

Typical RG6 has an operating voltage of about 300V. So I probably can't run a full 1.5KW through it. I have no interest in such high power anyway. This means I should be able to run RG6 on my transceivers so long as I don't use high power and everything (impedance) is matched fairly well.

I am already using RG6 on my HF radio the ts-520. This rig has an output matching network and easily accepts a 75 ohm load.

Modern rigs do not have this output matching luxury. Things also get a bit strange at VHF since wavelengths here are short. This brings us to transmission lines and how they behave as impedance transformers.

Consider this, I have a 2 meter ground plane antenna tuned for 146 Mhz and matched to 50 ohms at the feed point. If I connect RG6 75 ohm coax to this antenna and measure the swr at the radio it could be any thing from almost 1:1 to greater then 3:1 depending on antenna impedance. This is going to depend on the length of the coax. If my coax line is 1/2 wave length long it will mirror what is at the load and every thing will be happy. That is until you move frequency slightly in the 2 meter band. Everything is thrown off again. If your coax is 1/4 wave length long you have an impedance inverter and your swr will be very high. Any other lengths will give you impedances varying as I mentioned.

Of course any change in frequency especially at VHF/UHF changes the electrical length of your coax. At HF the wave lengths are large so this effect your may not see as the bands are only 300khz wide. You can tune the whole band without too much change in your coax electrical length. 

If this antenna was matched for 75 ohms and fed with RG6 you would suffer just the 1.5:1 mismatch. Some transceivers are okay with this and some aren't. Matching a ground plane antenna to 75 ohms is not easy. It will likely be less then that even if you turn it into a dipole shape or use a dipole. This is because a dipole in free space has a feed point impedance of around 73 ohms. Your antenna is unlikely to follow the free space model. Impedance normally goes down the closer to earth or any conductive objects it is. A ground plane antenna is just a dipole where the radials (or other leg of the dipole ) are used to match it's impedance to the feed line. The closer the radials to the main element the lower the impedance. Normal mag mount antennas are around 35 ohms due to the flat surface it capacitively couples to.

Alright, coax acts to transform impedance... We can use this I think. Enter the 1/12th wave transmission line transformer. For more information on this see:
https://www.cv.nrao.edu/~demerson/twelfth/twelfth.htm

Figuring out the length of 75 ohm and 50 ohm coax I needed for this was trivial. You have to take the cable's velocity factor into consideration but I think the tolerance does not have to be very tight as the bandwidth of this transformer is fairly wide.

Using a length of scrap 75 ohm coax and some scrap RG58 50 ohm coax I created what you see below. My lengths for 146 MHz target frequency was 3.57" for the 75ohm coax (VF: 82%) and 4.35" for the 50 ohm coax (VF: 66%). Basically the 75 ohm side of this transformer (BNC connector) is connected to the 50 ohm output of the radio and the 50 ohm side (F-Connector) of the transformer is connected to the 75 ohm feed line or load. This stabilizes the whole feed line network.

Testing with a home brew 75 ohm dummy load shows an SWR of near 1:1 across the 2 meter band.

AC9LF

Okay, so this will get me a 1.5 to 1 match with a ground plane antenna which is set at 50 ohms. Effectively this transformer stabilizes the SWR for the 2 meter band. Lets take this a step further. The J-Pole antenna. This interesting antenna is a half-wave end fed dipole made with copper pipe. As you move the feed point on a dipole from the center to approaching the end the feed point impedance increases. The J-Pole's lower J section is actually a matching transmission line stub. You match the antenna to your feed line by changing the feed point location on the J section. There are numerous calculators available such as this one: http://www.hamuniverse.com/jpole.html. I targeted a frequency of 146 MHz for mine and used mostly scrap 1/2" pipe for construction. This is what I have below:

AC9LF

Most RG6 shield is aluminum and easily breaks, Aluminum is also very difficult if not impossible to solder. for the feed point I used RG59 which is also 75 ohm coax with a bit more loss. The stuff I had was copper center, copper outer and PE insulation. I used stainless steel hose clamps to make for easy moving of the feed point location. Tuning er... matching was weird at first and I found out later through some experimentation that my coax air choke as shown was ineffective. The coax was still part of the radiating portion of the antenna.I got it matched 1:1 with 75 ohm feed line and the 1/12 wave transformer only to find out that when I moved the cable my SWR shot up above 3:1. This was because the connector pigtail was touching the antenna. That shouldn't happen. This was because of the ineffective ugly coax "BALUN". I attached a clamp on ferrite core right where the coax splits to the feed point and after moving the feed point around some more found the lowest SWR point. With the ferrite core attached it didn't matter if I touched the connector to the mast portion of the antenna or not. SWR/Impedance was now stable across the 2 meter band. I took my measurement bridge off and had a quick QSO with someone on a local 2 meter repeater with my HT. It used the 1/12 wave transformer to about 20 feet of RG6 connecting to the J-Pole pigtail. The J-pole was held up in a cat litter container.

Everything worked really well. Now I just have to mount this antenna outside which has the added benefit of the whole thing being able to get DC grounded. It is also a lot more rugged then my ground plane coat hanger hot glue antenna in the attic.

https://memegenerator.net/instance/66355681/all-the-things-yay-it-works

I have to admit it was a bit of work to avoid paying for premium coax cable. Nevertheless, this is where the fun and learning in amateur radio is! I have never before cut or soldered copper pipe. This has given me some experience in that now too.

I hope this little write up is helpful for anyone that stumbles across it.

73,
--
Nick, AC9LF