# Using 75 Ω instead of 50 Ω coax feed

Can I replace my old 50 Ω coax with RG6? It's cheaper and less loss at VHF/UHF. I would match by L/2 multiples of coax to repeat input and output impedance or L/C matching networks at each end.

Is this a viable alternative to expensive 50 Ω coax and what problems could I have?

So far I have used this on UHF and found a 6dB improvement in received signal.

• What bands do you want to use the 75 ohm cable on? Jun 24 '19 at 15:34
• The trouble with us is that we get a black box and just have to open it!! Not satisfied that the black box works when it is connected through some black cable to a black antenna ! We have to pull it apart and find out "why" it works? In the process we stuf fit up and it never works properly again! Then we kick ourselves for playing with it in the first place!!! Your question..what band? UHF . on HF any old bit of wet string works. Well sort of! Jun 25 '19 at 7:04

Sure, you can use 75 ohm coax instead of 50 ohm. In some cases (such as feeding a dipole), 75 ohm coax may be a better match to the load than 50 ohm. In other cases (such as feeding a vertical), it may be a worse match.

You may not even need to do additional matching. A 75 ohm load on a transmitter designed for a 50 ohm load is only a 1.5:1 SWR, which isn't that bad.

Do however keep in mind that in many cases 75 ohm coax is cheap not because 75 ohm coax is inherently cheaper, but because it's just plain cheap coax. The common application of CATV means big-box stores are quick to carry cheap 75 ohm coax because their customers care primarily about cost, and for a short CATV run, the high loss and poor isolation of a cheap cable may not be especially critical or noticed by a typical consumer.

For a transmitting radio application, the consequences may not be so benign. It is usually cheaper to buy better coax than it is to buy a bigger amplifier. And a directional antenna in a receive application fed with coax with poor isolation may pick up enough external noise to negate the benefits of the antenna directivity.

75 ohm coax will work, and you may be able to find good quality 75 ohm coax at a favorable price. But do try to make an informed decision.

• Yes thanks for that info! The specs for some RG6 are better than such things as R G 58 and I have found that the RG 6 I have for my sat TV has a lower loss than the 50 ohm cable I am currently using. That s not to say that one is better than the other, Just my current coax is years old and is crap! My question is! Should I just buy RG6 for everything, save money and not have to worry about impedance as long as can match everything? If a screwdriver in the back of a rig for a quick tweak saves me \$ then should I go for it? My antenna are all home brew so I can make them any impedance I want! Jun 26 '19 at 6:56
• @RJohnson If that's what you want to do, sure. Just keep in mind not all "RG6" has the same specs, and whatever matching methods you use will introduce some additional loss and cost of their own. Jun 26 '19 at 15:23
• RG-6 is closer in quality to RG-8. I wouldn't use RG-58 for any significant distance. Jan 5 '20 at 1:11
• You can get double shielded RG-6, I bought some once. Jan 8 '21 at 2:57

Many, many hams use 75Ω coax. I have for decades. All of my outdoor coax is RG-6 or RG-11. In the past I have used larger 75Ω CATV hardline, which has even lower loss.

Many hams just accept the 1.5:1 VSWR caused by the small impedance difference. I fed a 50Ω 2m array with 75Ω coax using a very simple matching arrangement, using two short pieces of coax at the feedpoint; one 50Ω and one 75Ω. (I can't think what it is called right now).

In fact, the feedpoint impedance of a center-fed dipole is 75Ω! It is only 50Ω at one low height. Feed a resonant dipole with 50Ω coax, and you actually have a small mismatch!

The power handing capability of RG-6 is almost identical to RG-213. And even at the legal limit, the voltage is still low enough that flashover will not occur between the center conductor and the shield even with a mismatch.

The thin copper plating on the RG6 cable centre conductor may be a problem at HF but for UHF it should be ok

K4KYV, myself, and others on the Topband reflector measured it. The consensus was that at 80 meters it made no difference, and at 160m it made a little difference but it was still insignificant.

This may be a duplicate question. In any case check out this search and see if you can find more answers there.

• Thanks for that!So much cheaper for RG 6 because it is mass produced! But matching between antenna and commercial 50ohm? Jun 24 '19 at 7:36
• RG-6 that I've used has a copper plate over steel center conductor. For HF I have concern that the copper is not thick enough to cover the whole penetration depth of the current (skin effect) which would cause much higher losses as current would be flowing through the steel. I used RG-11, but I have not used RG-6. How much power can RG-6 carry before it experiences arcing in the connectors when the insulation ends?
– cmm
Jun 24 '19 at 11:28
• However is it possible wi home brew antenna to make the impedence 75 ohmrather than59 and retune the tranceiver at the other end to look at a 75 ohmload? or is y=the intennal circuitry of modernrigs impervious to outside influences? Jun 24 '19 at 22:59
• Yes I think that if possible I reconfigure the rigs for 75 ohms and retune the antenna also. As most of my stuff is homebrew that is not a problem. Modern "black box" rigs are pretty unmovable but with such broadband coverage I wonder what the load impedance is on them anyway? How can they be rock solid 50 ohm over 130 to 500 mHz? Any way, theory aside, if It works, who am I to argue? Jun 24 '19 at 23:07
• MIke! I wonder if you ever looked at galvanise steel fence wier for LF use! That has to be cheap and readily available? Certainly easier to find than copper wire! Jun 25 '19 at 7:10

Jerry Sevick, W2FMI, described the use of multifilar-wound toroid cores to perform 1.56:1 transformations over wide HF frequency ranges. Given the vagaries of antennas as loads, this should provide a very close match between 50$$\Omega$$ and 75$$\Omega$$ coax - as long as the impedance at the input to the 75$$\Omega$$ coax is reasonably close to 75$$\Omega$$.

Page 65 of Understanding and Using Baluns and Ununs (2003) shows the implementation:

As shown in the photograph, four (4) quintafilar turns of thermaleze-coated wire are wound on a 1.4-in OD ferrite toroid core of $$\mu_r=250$$ (e.g., Fair-Rite 5952020801). Winding 7-8 is #14; the other four windings are #16.

The larger 1:1 Guanella balun in the upper portion of the photograph is not needed in unun applications, such as when matching a 50$$\Omega$$ system to 75$$\Omega$$ feedlines. Sevick measured this transformer to be essentially flat from 1.5MHz to 40MHz.

• There has got to be a simpler way! At UHF winding such things would be incossigle! I can use any feeder I want at HF but at UHF I put 5 watts into a line and get next to nothing out! That is why I am trying tv type el chepo coax! Because the tv satellite market uses heaps of the stuff it is readily available and cheap! Good quality 50 ohm coax costs a fortune! So using RG6 is a financially viable option provided that you can successfully match in and out! Jun 25 '19 at 6:42
• Thank you for that suggestion. I have used a simple pi network for the transformation, which is ok for single frequency use such as when I use one repeater. Your suggestion is more broadband , but I used what I had available. Never could understand toroids! My assumption would be . Wind 5 turns on one side and seven on the other and that would give a 50 to 70 ohm result? That does not work!!! So I am a Dill! But you can't fault the logic! Jun 25 '19 at 11:15
• I will bury my head in your suggested site and see if I can educate myself !! By the time I am old enough to know everything , No one will take any notice!! Jun 25 '19 at 12:14
• The loss per length on 75 ohm coax is lower than 50 ohm. The impedance mismatch loss at 1.5-1 is trivial. 0.18dB If the run is any significant length, you may be better off using 75 ohms and ignoring the mismatch. Just watch the max power rating of the coax. Jan 2 '20 at 22:44