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