Coax question

[Radio Wrangler]

Oh, G3ROO showed me a superb trick, sealing coax ends and antenna connections with hot melt glue.

For HF, the lowest loss feeder is open wire line, it's also the cheapest, and the most versatile for using an ATU. It suits balanced antennae, like a wire dipole over the top of the house (like mine) with open wire feeder into the shack, and a Z-match ATU circuit, I worked JH4UYB in Hiroshima on SSB two weeks ago with the radio only turned up to 50 Watts. So you can do a lot with nothing more than surplus wire.

David

[kellys_eye]

Just to throw a technical spanner in the works.....

I came across the attached article some years ago and just managed to find where I'd carefully (ahem) stored it for useful retreival at a later date.

[G0HZU_JMR]

Originally Posted by PETERg0rsq

Without knowing the antenna feed impedance (at any specific operating frequency), coax impedance is irrelevant and meaningless.

I agree but I think you may have a tough time convincing most radio amateurs.

The choice of 50R coax or 75R coax is pretty much irrelevant in this application if we assume both are of similar quality. However, I would still prefer to buy the 50R cable simply because it is more flexible.

Also, any surplus cable that is trimmed off can be used as general 50R patch leads, perhaps for VHF use etc.

For various reasons, I think I'd draw the line at making short patch leads from 75R cable. Too confusing for a start and I'm not that good at labelling things!

[Martin G7MRV]

There are many myths and fallacies in relation to coax, some of which have come out on this thread, although mostly the advice is sound,

when coaxial cables were being developed, there was contention over which set of parameters to design for - high power broadcasters needed cable that could cope with the power; this turned out to be a 33 ohm cable. Telecoms needed cable that had very low losses with weak signals; this turned out to be around 75 ohm. Extremely high voltage use warrented other impedences. At the time these cables were being developed, the military were beginning to go to transceiver architecture instead of seperate transmit and receive equipment. A compromise was needed between power handling and weak signal loss - 50 ohms!

Lets look at a typical amateur system starting at the transceiver -

The 'nominal' design impedence is 50 ohm, but it is likely to be only somewhere around this figure in a typical commercial grade transceiver. Hence most transceivers will also somewhere specify a maximum VSWR mismatch, typically 2:1, before the internal protection begins to fold back the output power. This equates to a possible mismatch far above that of 75 ohm coax. A 75 ohm load will create a VSWR of around 1.5:1, perfectly acceptable, and to be honest, 1.5:1 is likely to be the lowest you can measure within the accuracy of your SWR meter!

Now, lets say your transceiver actually is a pure 50 ohm resistive impedance (not likely!) and you use 75 ohm coaxial cable to a 75 ohm purely resistive load - you wont notice the difference! True, the forward power will be a little lower than expected, but in a typical amateur system, and especially at foundation license power levels, the loss of around 1dB is not noticable in contact.

Add now the antenna. I challenge most amateurs to have an antenna that, without the use of matching networks, comes anywhere near 50 ohm, or in most cases even 75 ohm!
A dipole is considered 75 ohm, but this holds only for a resonant dipole, perfectly horizontal and at greater than 1/2 wavelength above the ground. Lower, and the impedence drops, make it an inverted V, and the impedence drops (as does the resonant frequency). A resonant dipole at normal amateur heights for say, 40m, with a bit of a V, could well measure closer to 50ohm than 75, likely closer to 30! It will also no longer be a pure resistive impedence, you now have reactance to contend with.

Impedence, in a typical amateur system, is really of little consequence in itself. However, coax when used away form its characteristic impedence increases in loss rapidly. This is the big problem. If your transceiver is 50 ohm, and your antenna is actually 50 ohm, then at the frequency at which that antenna present that impedence, the 75 ohm coax will do little more than add a slight impedence bump at both ends. These two bumped will create conjugate match reflections and ultimately all the energy will radiate.

If however, the antenna is not 75 ohm, as in the case say of using a G5RV design, the losses will increase. At this point, matching the antenna becomes critical. Using a remote ATU would allow the antenna to be matched and the losses kept low in a 75 ohm line. If the antenna is a single frequency design, use of an electrical 1/2 wavelength of 75 ohm line will create the situation where the line is 'transparent' to the network - the transceiver will 'see' the antenna impedence. I actually use Times TFC-10 75ohm cable for VHF work on field days, using a carefully cut 1/2 wavelength multiple.

So, in simple terms - the impedence is, in a typical amateur system, largely irrelevant, but the losses when uses with a non resonant antenna can be excessive.

Now, onto more critical problems. These have already been alluded to by others - quality and dimensions.

Typical TV grade, even satellite grade, coax is quite simply junk. Only the highest quality, double shielded with greater than 95% braid is worth using, otherwise its loss and leakage is too poor. Trouble is, once you get to the really good stuff, your at similar prices to reasonable 50 ohm coax. To be sure though, much RG-58 out there is also dire!

Most TV coax also has a solid core, and either light foam or aircell dielectric - both will distort on bending and both will melt with too much power. Remember as well, that if your system is mismatched, these losses are multiplied. The solid core means its liable to break with too much flexing around connectors.

Now, finding connectors for standard amateur types that will fit TV type coax will be difficult.

On the whole, the electrical differences are irrelevant, but the physical dimensions and construction arguments mean using TV type cables is largely a false economy. I get away using the TFC-10 for the twin reasons that it A)has dimensions close to those of RG-213, and B) I got a drum for nothing.

Although we know what your transceiver is, and it can certainly cope with 75 ohm cable, better in fact than most due to its PI tank output network, you havent told us what antenna your planning! I would suggest you decide which band(s) you wish to work, and build an antenna to suit, and then decide on feeder to complete the system.

If you go with a G5RV, or even better a ZS6BKW, consider setting up such that you can bring the ladder line section as close as possible to the rig, so you use the minimum of coax. A remote ATU can allow matching at the start of the ladder line section (although they are expensive). If you can get the ladder line to the shack, using just a few feet of coax to bring the feed in to the ATU, that will give you the least loss.

Personally, I would suggest picking one band to start, putting up a dipole, using whatever coax you wish, but just making sure your dipole is resonant, and getting on air! Your loss in that case will not be of much consequence.

As a rule - it is said that you should put 10% of your budget into the radio, and 90% into the antenna system! Buy the best quality you can afford, and this particularly applies to coax and connectors!

[Radio Wrangler]

The best place for an antenna tuning unit is right at the feedpoint of the active antenna unit.

This normally means high up in the air, giving all sorts of problems about remote control, support of its weight as well a s weather-proofing. So, compromise is needed.

Putting the ATU in your shack gets it in out of the weather, doesn't need remote control, and a bit of table or bench space fixes the support issue. The feeder is the downside of the deal.

Running a length of feeder in a mismatched condition multiplies its losses. So any reduction in the lossy-ness of the feeder pays back very favourably. If you're concerned about the relative costs of cheap 75 and 50 Ohm cables, they you're probably going to be thinking of wire antennae. Towers and aluminium Yagis make cable costs vanish into insignificance.

Losses in cables come in two sorts. Dielectric losses which are worst in places where the cable voltage is greatest, and conductor losses which are worst where the cable's current is greatest. As you tune around different bands and frequencies, the locations of these maxima move around, but with a mismatched antenna you have to assume they'll be somewhere.

Dielectric losses can be trivial at HF, unless you have wet material between the conductors. Dry coax, ribbon line with slots in it, or open wire feeder fix this.

Conductor losses are the ones to worry about. This is why cheap coax with minimum copper content is lousy. Losses turn RF power into heat, and polythene dielectric is rather meltable.

So if you are tied to a basic wire antenna due to either cost or location you'll find that something reasonably balanced has advantages, and that open feeder back to an ATU works as well as anything.

Now we come to the inconvenience. Your Pi tank in your TX can match a fair range of impedances, but it's unbalanced. But if you use a balanced antenna and balanced feeder - which also help a lot with reducing the pick-up of all the RF interference spewing out of modern homes, you'll need to convert from balanced to unbalanced somehow.

OK so there are balun transformers, but these really only work well over a smaller range of impedance than simple antennae present if you want to work more than one HF band.

The solution is to combine the balun and ATU functions into an ATU circuit with a balanced port for the antenna and an unbalanced port for the radio. There have been several designs published, but they all mean winding some coils and finding some variable capacitors with large spacing between their plates. These things are in demand, and that costs money.

The worst part of doing this with a valved transmitter is that you now have effectively two adjustable impedance transforming devices, one after the other.

The usual approach is to switch the radio into a dummy load resistor ( 50 Ohms that will take 100W for several tens of seconds) and then tune the balanced ATU to make the antenna look like 50 Ohms.

This gets tedious after a while, but it will get you on the air with the best signal you can manage without spending on steelwork.

David