RG58 / RG213 and SWR

I've got a setup that works well so far: 1/2 lambda silver rod, 10m of RG213, window cable thing, another 3m of RG58 into the SWR meter and then rig.

I also have a 15.5m RG58 that I wanted to use to be able to move the rigs around when the normal place is occupied, or even onto the terrace in summer.

I set that all up, replacing the 3m RG58 with 15.5m RG58 but measured SWR that pegged the needle all the way right with no adjustments. I was thinking that just replacing one cable would have little/no impact on SWR, let alone put it off the scale.

To be sure, I checked there was no short in the long RG58 and it is OK with a continuity meter. No apparent issues. I didn't check anything else, but the cable is new and connectors seem fine.

The next step was to connect the long RG58 from SWR meter direct into the silver rod to see if was anything in between - but I got the same result - SWR pegs totally off the scale.

So Im guessing either something is up with the long RG58, or the antenna is completely not matched with it due to the new lengths.

Any suggestions what I could try next? Thanks!

p.s. I did want this longer cable also to connect a mobile dipole, but until I know it's 'good' there is little point. Last resort might be to turn it into a T2LT!

Edit: it's RG58 C/U if that makes any difference?

hi Ian ,if reading your post right.
hi swr with long rg58
ok with other length of rg213.
really sounds like rg58 is naff.it still will pass a short test if damaged inside.
the length of coax shouldn't matter from the silver rod, in fact a long length of rg58 would mask hi swr
hope this makes sense
regards mark

+1 on the coax being naff.
Has it got any kinks in it as that will damage it but not show a short on a test meter.

Thanks guys. I'll double check for kinks etc. It's unused for anything else so far, so would be surprised/disappointed if it's dud.

I don't have much electronic gear, no dummy load either.

What happens when the core is damaged, does that mean split, so stuff 'leaks' out?

Think it throws the impedance out so the swr will never be correct.
Somebody will put us both right soon lol.

There's more to coax than a piece of wire to connect your radio to the antenna. 

It doesn't help that we shorten the word to 'coax' and forget the meaning of co - axial with most having very little understanding or knowledge of the theory. 

Not surprising really, have you seen the formulae involved?!!

https://en.m.wikipedia.org/wiki/Coaxial_cable


Even if you grasp a little of the theory you'll soon realise that the stuff is rather delicate and any crush, kink or pinch can literally ruin it. Stretching, yanking, pulling are other common abuses to coax, (it's NOT a piece of rope), which can also ruin it. A heck of a lot of people can't even terminate it properly including 'seasoned' radio operators bunging on end connectors with gay abandon. 

A multimeter is next to useless for checking your coax apart from seeing if one wire of that braid strayed thereby causing a short or if a poor attempt at terminating a connector left you open circuit. (That stray wire short is the commonest problem!)

The only real way of testing your coaxial cable is by using 'Time Domain Reflectometry', something most of us mere mortals never venture upon.



My take on the question?

15 metres of coax? Multiplied by the 'velocity factor'.......getting rather close to 11m and a full 'wavelength' aren't we?
Full wavelengths have rather large impedances so of course a SWR through the roof....especially if the coax has become the 'antenna'! 

Shorten or lengthen the coax and the problem appears to alleviate but we aren't really fixing the problem. SWR doesn't tell us the whole story.


By all means check the coax connectors for shorts or open circuit with a multimeter as 'terminating' coax isn't as straight forward as it seems. Check the antenna out too, not with the meter as matching coils in antennas are usually DC shorts, (not at the designed RF), but for obvious visual damage or corrosion....

.....and of course you can utilise the old favourite of winding an air choke using the coax at the antenna end. 



Radio is a 'fun' hobby, just when we think we're doing well something throws a spanner in the works and we have to learn a bit more. I'm always impressed with the operator who has more test gear than radios. 


More of my waffle but all in the spirit of helping.

All the best,
Victor

Victor wrote:

There's more to coax than a piece of wire to connect your radio to the antenna. 

...

15 metres of coax? Multiplied by the 'velocity factor'.......getting rather close to 11m and a full 'wavelength' aren't we?
Full wavelengths have rather large impedances so of course a SWR through the roof....especially if the coax has become the 'antenna'! 

...

Radio is a 'fun' hobby, just when we think we're doing well something throws a spanner in the works and we have to learn a bit more. I'm always impressed with the operator who has more test gear than radios. 


More of my waffle but all in the spirit of helping.

Thanks Victor! All waffle greatly accepted at this end!

Funny you mentioned the coax becoming the antenna as when I put everything back how it was, I checked the SWR on the original system and suddenly it s way higher than before! Then I remembered I had not swapped the coax at the antenna end so I was running a 15m coax with no silver rod - doh! But the SWR in this case was only about 5:1. So yes all this is crazy stuff.

Further to the 'coax as antenna' if I don't get the figure what's up then it is a good excuse to muck about further and make a T2LT from it. Nothing to lose there I guess except some time spent.

You're more than welcome Ian, I am still learning things everyday myself.
(Hoping the learning never stops as it's so enjoyable.)


I'm very much a fan of learning by experience as well as reading to further understand, but honestly only take it so far. It's one thing to be aware of CMC or Common Mode Currents and such like, but knowing every intrinsic detail becomes rather boring and unnecessary.

A T2LT or sleeved dipole, home constructed can be a great way of learning, but I give fair warning that it can also be frustrating. They usually work first time but when they don't you'll chase your tail as to why not! (Most start all over again and either rewarded with success or give up in failure.) 

I built one recently for VHF work and had the clever idea of using stub matching and a colinear section to make it a VHF/UHF dual bander.....built the damn thing three times before I got it right!!
(Used an 'ugly balun' or air wound coax choke to tame that bugger too. )


Might be worth giving an air wound choke a shot first, it'll only take a bit of time to essentially coil up some coax at the antenna end. Pretty much what you'd be doing with a T2LT anyway.


Discoveries aren't usually heralded with the exclamation of "Eureka!", but with the words " Woah....that's weird".
It's the "Why did it do that?" pursuit that makes it all fun.


Keep enjoying yourself mate and always ask questions. It's how we learn, grow and keep our mind healthy.
(But don't be afraid of the 'bores' who've succumbed to the facts & figures!)


All the best,
Victor

Captain Fantastic wrote:

I don't have much electronic gear, no dummy load either.

What happens when the core is damaged, does that mean split, so stuff 'leaks' out?

Na, nothing messy, but a few final stage transistors have taken a nosedive from that.
One last thought.

I had heard of a Ebay sleeze ball or two selling cheap 'unbadged' 75 ohm coax as 50 ohm in the past.

So nowadays, I reckon the same thing could be a danger.

Hi Paul,

We had a topic of conversation a while back about 75 ohm coax. The worst it'll do is give you a SWR of 1.5 on a 50 ohm load so still running at 96% efficiency and it's easy to match with a 1/12th wave section for single band use or don't really worry about it. I use RG6 75 ohm on everything from low HF up to VHF on my setups without a problem.

It's cheaper than the 50 ohm stuff us weird radio lot use as they make and sell so much more of it. It's usually much better quality as well despite the cheap price and often 100% screened to boot. Just a sod if you're used to soldering braid to PL259's.
(I use compression fittings.)


I agree though that it's totally dishonest to try and flog it as 50 ohm and bumping up the price as such!

Just to chip in with the least technical possibilities, drawn from personal experience, sometimes it's the simplest things that screw it up...

Check the centre connectors of the plugs, sometimes the solder 'tip' is well attached to the conductor wire, but totally disjoined from the actual prong on the plug, or vice-versa the wire has separated from behind the solder. Either way it can be dead/intermittent/impaired but still looks good on quick inspection.

And a real stinker that caught me out - pay close attention if using those 'double female' coax joiners. I have a couple and one of them is pretty short length. If your plug terminations are elongated (mine sometimes have a long solder nipples on, lol) it means they can actually butt together inside the short barrel joiner and prevent the shield from making good contact.  The threads engage but the faces don't join up hence the RF continuity is degraded with measurable high SWR. Well it happened to me anyway...

Great point from Victor on 75 ohm coax. A lovely simple balanced dipole runs way closer to 75 than our radio's preferred 50, so I would see 50/75 coax as equal contenders in that std-reference battle.  Given the accuracy of their gain figures I'd say a lot of manufacturers might be hopelessly off target with their 50 ohm matching circuits anyway, so using 75 coax might make for a better circuit on half of the coin tosses

Best of luck resolving the issue bud!

Nice pointers there Chris.  

I mentioned how even the 'seasoned' radio operator can still screw up terminations and sometimes it's due to the terrible quality plugs they're using. 

The adapters you mention can be equally appalling and not fit for purpose. Frustratingly so.
(I got caught out with a measuring T-adapter, shouldn't have bought a cheap one!)


Speaking of the 50/75 ohm mention, I have a dipole for 2m, (75 ohm or about), 75 ohm RG6 running to it and a 50 ohm ohm radio running on it. In theory the SWR should be >=1.5, yet my SWR is less than 1.2 across the band.

We unfortunately suffer from that Pavlov classic conditioning.....where cheap is bad.

Price wise, cheap connectors are terrible but you can go 'cheap' on coax and antennas yet still do a fine job.