Using 75ohm coax for TX antenna

[XXL]

Hi, considering a dipole is 73ohms at the feedpoint, would there be much loss using 75ohm coax to feed it from the rig ?

[Albert H]

What's the output impedance of your transmitter?

Also, you need to consider that a dipole - whilst it's ~72Ω, you'll find that it needs a balanced feed. You can't just attach your coax to it and hope - the best match you could get would be about 1.5 : 1, so you'd be wasting power.

The easiest way to make a balanced feed for a dipole is to use a Pawsey Stub (look it up!).

Years ago, all my link gear was designed for 75Ω - both transmitters and receivers - and I used the cheap brown "low loss" TV 75Ω coax. The aerials I used were TV aerials (usually multi-element Band IV Yagis) and my links were usually in the lower part of the UHF TV Band. There were tthree big advantages to this - TV aerials with insanely high gain are cheaply available form any branch of B&Q, a TV aerial never looks out of place, so is unlikely to be sussed, and nobody else was building UHF gear, so I had the whole UHF link market to myself and made a hell of a lot of money!

[radium98]

Albert H are you rich enough , i dont think so but i hope that at the end of the age , btw never seen any of the design you always speak off , give us only few picture of one of thoses mistery. I love to see what you did once plz , dont take it offense , but you never show us a treasure .

[XXL]

@albert obviously the rig output is 50ohms. No one makes 75ohm outputs. I know how a dipole works and I use pawsey stubs. Now that’s cleared up will it work just fine ?

[Albert H]

Not really - you'll have a mismatch at the TX output - it'll be looking into a 75Ω load instead of a 50Ω load. There are ways of doing the impedance transformation, but all of them are lossy.

[Albert H]

Radium:

Here's the kind of little UHF receiver and VHF exciter - inderneath the PCB, there's a National dual PLL IC, with one half used for the receiver and the other half used for the transmitter. This board would put out about 1 Watt on Band II, and the receive sensitivity was around 2µV for a fully noise-quietened signal.

remote-end.jpg

They were tricky to align, but worked well enough when set up.

[mpx]

XXL,

You can use a twelfth-wave coaxial transformer made from a piece of 75ohm and a piece of 50ohm cable to match the impedances together.

At the output of the TX, use a short piece of 75ohm coax followed by a short piece of 50ohm coax, and then onto the main feeder, in this case, the 75 ohm coax.

75 ohms impedance to 50 ohms is a ratio of 75 ÷ 50 = 1.5, a phase shift of 29.334 degrees which equals a wavelength of 0.0815, almost one-twelfth wavelength.

The physical length of the matching coax cables is dependent on the velocity factor, which is mostly dictated by the dielectric insulator around the center core. Solid polyethylene (PE) is commonly used for RG213, RG58, RG59, RG11 and gives a velocity factor of 0.66, air-spaced PE or foam dielectrics have VFs closer to 0.8-0.85, commonly used WF100, for example, is 0.81

So for each piece of matching section: 0.0815 × Wavelength × velocity factor

Example, using RG213 and RG11 at 88MHz: 0.0815 * 3.409m * 0.66 = 0.183m = 18.3 cm

Solder the two short pieces of coax together, heat-shrink and seal them. Put a coax plug on the 75ohm coax end to the TX and a socket on the 50ohm end, or solder the 50ohm to the main 75 ohm feeder.

The 50 to 75 ohm mismatch loss is around 0.18dB.
Coax losses for the transformer would be around 0.02dB if using RG213 and RG11.
You could use coax connectors and couplers to connect it all together. These will have some loss though, and I would avoid PL259/SO239 if possible as there can be as much as 0.8dB loss per connector. Use BNC, TNC or N connectors.

You'll want a 1:1 balun at the antenna feed-point to go from unbalanced coax to an open dipole that is balanced.
Pawsey Stub is the simplest and best solution, as Albert suggested.

Consider the losses involved in the coax. I would avoid thin cable such as RG58 and RG59 unless for really short runs as they are very lossy. And anything above a few tens of watts will warm the coax. I wouldn't go above 50 watts on anything thinner than 10mm OD, check the VSWR is good and check the temperature of the cable every few minutes to make sure it's not melting..

[Frequent Lee]

I can remember the band V links that Stephen used to supply in the late 80s, we linked for miles on just a few mW with a vertically polarised TV aerial on tx/Rx ends.

[XXL]

Not really - you'll have a mismatch at the TX output - it'll be looking into a 75Ω load instead of a 50Ω load. There are ways of doing the impedance transformation, but all of them are lossy.

Your kind of missing the point though. A dipole is 73ohms anyway. So a 73ohms dipole after a 50ohm coax cable is still a mismatch.

So why not just use 75ohm coax into a 73 ohm dipole ?

[MiXiN]

I have a 2 element Yagi here (effectively, it's just a Dipole with 1 director). I also have a 3 element Yagi, but it still needs to be assembled.

Both use what's called an "hairpin match" to create a 50 ohm impedance.

I've purchased a few Yagi antenna's in the past, and all the manufacturers say that the hairpin match is much less lossy than the Gamma match. I'll take their word for it, as they're all reputable companies (Innovaantenna, Superyagi, Smartech Italy).

Here is an image of the hairpin match part of the 2 element Yagi I have; it's basically a DC short across the inner & outer conductor, with a shorting bar that slides up or down to do the impedance matching.

[mpx]

A 2-element yagi will have an impedance fairly close to 50 ohms. A 3-element slightly less, probably around 35 ohms

[radionortheast]

If its only a short distance its low power theres no reason not to use either the 5mm 50ohm or the sightly thicker 75ohm brown or white coax. I found both the 50 ohm with a pawsey stub, then 75 ohms, worked the same, i’ve only ever used a few watts, I would use a trap about a quarter wavelenght away afew turns of the coax from the 75 ohms, you can do the same with 50ohms if you don’t have a balun, the trap is important it stops the signal traveling back down the lead. With a 0.1w transmitter it wouldn’t matter so much, it might effect the signal, you probably wouldn’t beable to hear it as far away without a trap, you might get lucky not get anything coming back down the lead, kind of like using an off the shelf receive antenna its kind of pot luck thing.
You have to have something to measure the signal with, I would often use a scanner with the aerial unplugged, there might be something better to measure the field with. I did the same with a gp antenna too, made a 75ohms version the radials looked like a bell. As long as the signal isn’t coming back down the lead, its only the dimensions of the aerial that will effect the signal. Dipoles ground planes work good in a reciever too, I don’t expect anyone out there is into recieiving.