Coax question

[ITAM805]

After years of being a SW listener I finally decided to join in and gained my foundation licence last week! So soon I'll be looking to fire up my vintage Trio TS-520 (at 10W of course) to speak to the world!

My question is, what do folk think about using this 75Ω stuff in place of the recommended and expensive RG8/58 stuff? I have read and been told contradictory advice saying it's too lossy, it doesn't matter and so on

Being a cheapskate I dont want to spend out on the pricey coax if I can away with the cheapo stuff!

You won't get away with it, best case is low signals (both in and out) worst case a blown PA, RG58 isn't expensive look on ebay.

[lesmw0sec]

TV coax is not recommended for the higher bands, but will work perfectly well on 160M with o/p power up to 100W. Rather poor in mechanical terms (soft) compared with the proper stuff though.

Les.

[Nicklyons2]

Don't; wrong impedance = poor match also fitting the usual RF connectors (PL259 and 50 Ohm BNC) will be more fiddly because the wire diameters aren't correct. RG58 can be obtained fairly cheaply if you look around as can UR76 which is a good robust cable of 50 Ohm impedance.

[Al (astral highway)]

Hi there, Amazon even sell RG58 coax, approx £8 for 20metres. That's got to be affordable compared to the cost of going mismatched..?

[G0HZU_JMR]

For your application (TS520 10W HF bands and some kind of multi band aerial) the technical differences between 50R and 75R coax are largely irrelevant in terms of what signal performance you will achieve. Any difference either way will usually be tiny and I think your TS520 at 10W will be fine with it. However, a lot depends on what type or aerial you use and how many bands it operates on.

But I think you will discover that it will prove a false economy in the long term to use the 75R coax. This is because the cable isn't as pliable as RG58 and also it might not be as easy to fit the connectors. However, something like a PL259 will be more forgiving than other types of connector.

Most radio hams love to play with aerials and so a typical run of RG58 coax could get uncoiled and deployed several times in several years and it is usually shown little sympathy during these redeployments. You can coil it up, throw it in a box and use it for a portable station on a hill then take it home and use it somewhere on the roof or in the garden. You could do the above several times a year for several years because the centre core of the coax is multistranded and is very flexible

I don't think 75R hard cored satellite coax will be as pliable because the centre strand is a single hard core and I'd expect it to be less reliable over time unless you fit it and don't move it very much.

[PETERg0rsq]

In an ideal world, to have optimum power transfare (transmit, as well as receive signal) you require a system with matching impedance.

Your radio is 50 Ohm, so ideally you would connect it to a 50 Ohm antenna with 50 ohm coax....perfect

However we are not in a perfect world! Your radio is 50 Ohm impedance, but what is the antenna impedance? I doubt it is 50 Ohm.

The next best scenario is to connect 50 Ohm coax to the radio, and take it to the antenna, and use a "matching network" (or antenna tuner!) up at the antenna to match the 50 ohm coax to the antenna impedance. Not the most convenient if you want to re-tune the system.

However what most people do is put the "matching network" or antenna tuner straight after the radio, and tune the system there, so now you have your 50 ohm radio connected to a 50 ohm input of the antenna tuner, which then "matches" the radio to the impedance of the coax/antenna system.
You will still have a large SWR on the coax caused by the mismatch between the antenna and the 50 ohm coax, with all the losses and potential for causing interference that it generates. All that is happening is you are making the radio "see" a 50 ohm load, but the rest of the system is still mismatched. So no real gain in using 50 Ohm coax from the tuner to the antenna if your antenna is not 50 Ohms across all frequencies you plan to use

Now if your antenna has an impedance of 75 Ohm, then using 75 Ohm coax, and a tuner at the radio end is the perfect way to go!!!......But what is the impedance of your antenna?....and it could change as you change operating frequency!!!

Finaly losses at HF due to coax mismatch are quite minimal, even a 2:1 or 3:1 SWR will only result in a few dB signal loss in a medium length cable, so if the radio can see a 50 ohm load (using an antenna tuner straight after the radio) does the rest of the system really matter?

So the answer to your question is......

It depends


To try and help more:-

What antenna are you planning to use, and what frequencies do you plan to operate on?


Also I would suggest it is better to use good quality 75 Ohm coax, than cheap poor quality 50 Ohm coax, though there are certain practicalities surrounding using 75 ohm coax as mentioned in a previous post. (Choke balun's can mitigate the issues of using mismatched coax if understood and used correctly)

Dont cut costs on coax, if you get good cable, and look after it it will last you many 10's of years, cheap cable will be lossy (75 or 50 ohm) with less signal to and from the antenna, can result in noise in your received signal from local sources, and leakage of transmitted signal (potential for causing local interference) and will deteriorate quickly.

Also for HF use, especially if using wire antennas, i would avoid coax altogether for most installations, and use open wire feeder, or ribbon feeder if practical. Though this may be a little advanced for a new license holder at the moment.

Peter

PS if you plan to operate on VHF and higher frequencies, then a lot of the above no longer applies!!!

[Sean Williams]

The main reason that good coax is expensive is down to the quantity of copper involved.

Screening on most 75r "tv" grade coax is quite frankly worthless.

Therefore you may find problems with RF leakage, in both directions....

Remember, your 10Watts will get partially absorbed due to losses and mismatch, the situation is reciprocal, the signals you want to listen to will also be absorbed as well.....

While at HF matching isn't as critical, why waste time and effort with a heavily compromised system?

[lesmw0sec]

Originally Posted by PETERg0rsq

Also for HF use, especially if using wire antennas, i would avoid coax altogether for most installations, and use open wire feeder, or ribbon feeder if practical. Though this may be a little advanced for a new license holder at the moment.

Very good advice. The open wire or ladder line is very low loss and if you have a clear run to the TX, it would be a good choice - especially as it is a lot cheaper! The only point to remember is that it does not work well if coiled up, bent at odd angles or buried, or fed through thick walls.

If you use this to feed a long doublet and use a matching unit (ATU), you can also adjust the system to operate well over a number of bands.

Les.

[llama]

The September 2014 edition of RadCom has an article on page 68, highlighted on the front cover as "Beware cheap coax".

I'm no expert, but the gist seems to be that there are no real standards for RG8/58 since the military abandoned the designation in favour of RG213.

Maybe someone with more expertise than me and access to the article can interpret the words in the context of the OP's requirements.
Graham

[dodgy-dxer]

It might be useful if someone actually looked up the specification of the transceiver they are making comments on!

The TS520 is a 70s rig with a valve PA that uses a PI output MATCHING network. It is not "designed" for 50 ohms +/- 0 and nothing else

It actually specifies "50 ~ 75 ohms" as its "RF output impedance"

http://www.kg6haf.com/downloads/520_Manual.pdf on page 4

So I ask "what mismatch"?

I was first licenced in the 70s. MANY people used "telly coax" to feed a dipole which rarely had a balun and matched fine into a valved PA with a PI network at the output. SWR was rarely worried about and was not an obsession it seems to be now. No-one's rig "blew up" and we worked all over the world.

A dose of reality is needed

73 Fred
A65BD

Originally Posted by ITAM805

in place of the recommended and expensive RG8/58 stuff?

I think we where working on that assumption.

[Restoration73]

There seem to be too many generalisations on coax. CT100 is an excellent sat coax, with
a substantial copper braid and foil, unlike most that DIY outlets sell, very little braid and
metallized plastic foil screen. 75 ohm is fine for a straight dipole. UR67 (RG-213) may well handle 16kW at 30MHz, but only 500W at 144 MHz.
I was given a load of 5D-2V 50 ohm coax with a foam dielectric, although thinner has a
lower loss than UR67, with the only inconvenience the use of adapters to fit standard
PL-259 and N plugs.
You will probably find the pi tank on the rig will tune up a lump of wire or even a 6" nail!

[Sean Williams]

Fred, with respect, I was trying to avoid drawing attention to the finals in this radio - the OP is going to attempt to run the radio at 10W output - AFAIK this is not really possible while maintaining correct tuning of the finals and driver stage - it is questionable that this radio is really suitable for a novice operator, given the restrictions of the FL.

While I agree that this radio with a fully adjustable PI output stage will match to a 75r system, it would be wrong not to offer advice to aim for the nominal 50r impedance that is almost universal in all equipment available today. Incorrect tuning of the finals could then mean that the PA stage operates as a doubler or tripler, thus allowing the operator to operate out of band without knowing it.

In the case of a new foundation license holder it is up to more experienced operators to offer advice that is in the best interest of the OP - doing everything we can to help will mean that the possibility of failure, and or poor results is minimised, and then hopefully the OP will then be encouraged to go further with the self training aspect of the hobby and progress to a level where they are confident and competent to make these sort of decisions off their own back.

Incidentally - CT100 from Maplin is £2.29 a metre, whereas RG58 is only £1.49 - therefore the "correct" cable is cheaper......

If the OP really cannot run to some decent quality coax (that will fit the connectors without faffing around), I may have some to spare here.

Whatever the OP chooses to use, the reality check is still simple - every station accessory sold today is "designed" for 50r sure a 1.5:1 VSWR is not a big problem, but with QRP operation is it not the greater reality that optimisation of performance of the complete antenna system will give the greatest payback on investment?

[PETERg0rsq]

Originally Posted by dodgy-dxer

So I ask "what mismatch"?

The mismatch will occur between the coax and the antenna (unless the antenna is 50 (or 75) ohm at all operating frequencies. If such an antenna exists I want one ).

This will create a mismatch regardless of what impedance coax is used.

50 ohm coax is only a good choice when you know the source and load impedance are both 50 Ohms!

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

What is more important is the "RF" quality of the cable (low attenuation at the frequencies in use, and good ability to keep the RF energy inside the cable), and a good understanding of common mode current in the outer screening.

Unfortunately for the OP, good RF cable (regardless of its impedance) is expensive.

Good quality 75 ohm satellite coax is always going to be a better RF cable than cheap e**y RG58.

Peter

[ITAM805]

Many thanks for the replies chaps - even if theyr'e contradictory

I should have given better information in my post, my apologies. I have the 'el chepo' Screwfix stuff already plumbed in to the 520, which I'm only using as a receiver currently, I removed the ptt to avoid the temptation So I wanted to know if I needed to redo the cable run with some different coax. This would be a chore but not the end of world as we know it.

Regarding my licence status and the 520, there are at least a couple of mods which allow one to control the power. For instance, this can be done by lowering the screen voltage on the 6146 PA valves apparently.

Also I should say, that I'm a member of the local radio group and they are happy to assist me in my endeavour. And of course, there are you wonderful folks always ready and willing to help

[Sean Williams]

Of course you could modify the transceiver to suit, but as a FL technically you are not permitted to use modified equipment......

Incidentally, if you modify the circuit, and the output impedance of the tank circuit changes (which it will as a function of the anode current falling) the original PI circuit may well not match the anode impedance down to the rated output impedance.

I would certainly replace the "el cheapo" stuff with something at least half decent, you want your RF to make it to the antenna, and not radiate around the shack and house....

[ITAM805]

Of course you could modify the transceiver to suit, but as a FL technically you are not permitted to use modified equipment......

I haven't got an answer to that Sean

[Sean Williams]

It's more a case of being sensible - and how you read the terms of BR68F - well off topic for this thread though.

I doubt anyone will really bother what you do from a regulatory point of view.

[Radio Wrangler]

Something no-one has mentioned is that whatever coax you use, make sure water can't get into it. It draws the stuff along like wick, and it corrodes the copper braid. Once that happens, the loss goes up dramatically.

Everybody's covered the tuning range of your Pi tank.

At the other end, 75 Ohms is close to the impedance of a half-wave dipole. No antenna is a natural 50 Ohms, so they all have to have some sort of impedance transforming arrangement if they are supposed to be a good match to 50 Ohm coax.

Cheap and nasty 75 Ohm coax is lossy. So is cheap and nasty or expensive and nasty 50 Ohm stuff. Whatever you use is going to be similar in price if it is any good because you'll have about the same weight in copper in it.

That said, for a given amount of copper per metre, mathematically, 75 Ohm can be slightly lower loss. It's why all the telecomms stuff, including all the analogue coax submarine cables are 75 Ohms.

And on the matter of transceivers, there seems to be an awful lot less 10W commercially made radios in the UK than there are Foundation licences. I can only conclude that no-one is causing any noticeable trouble.

I don't suppose it'll be too long before you go for an upgrade, anyway.

David