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Markone
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PostSubject: coax problems   coax problems Icon_minitimeThu Jun 25, 2020 10:13 am

hi all hope your well
need a little help,just up graded 25m of rg58 for 10m of rg213 and swr gone from 1.2 upto 2.2.
antenna is a silver rod set up as instructed and 1.2 out of the box great,put it on a pole on gable end and 1.2,great.thought id replace the coax with 213 ready for a better antenna in coming months but swr now 2.2
now got this down to 1.2 with zetagi matchers but watts on meter now dropped from 40w to 25w,what the hell is happening,the 213 was new from knight with plugs fitted.
heard about coax being different lenths but no probs when first set up
read about coax being multiples of 11 feet,10 metres is about that.hope ive made sence
regards mark
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PostSubject: Re: coax problems   coax problems Icon_minitimeThu Jun 25, 2020 9:03 pm

Have you tried a choke near the aerial? 4 1/2 turns on 4 1/4 inch plastic former may help, or there's ferrite beads/rings you could try. Look up 'common mode current' to see what I mean. That could be your problem. Have you got different swr at band edges? That'll give you an idea of tuning. Or you could just use 10 metres of RG58 if it's long enough to reach.

RG58 has a lot of losses anyway, I measured 20 metres and got 30% loss, so swr will be out due to the losses. I plan to replace mine with RG213 (got loads of it) but I may also change the aerial too. I need 30+ metres for mine (around the garden, not straight across the grass). I've just got a nanovna (cheap but fancy analyzer) too so that'll be fun setting it up.
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PostSubject: Re: coax problems   coax problems Icon_minitimeFri Jun 26, 2020 6:01 am

I had the same problem when I changed from RG58 to RG213.....the SWR went up high. I now use RG8 mini coax which has the same specs as RG213 but is much easier to use as its thinner and more flexible. Since I changed to RG8 mini the SWR has returned to normal (low). See link below.

https://www.nevadaradio.co.uk/product/nevada-rg-mini-8-super-xx-per-length/


Last edited by Pagan on Fri Jun 26, 2020 9:00 am; edited 2 times in total

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PostSubject: Re: coax problems   coax problems Icon_minitimeFri Jun 26, 2020 8:46 am

I use RG8X And it goes round bends very easily and the swr over all band its very good a little high at the ends of the band but still very usable, with better coax you might need to adjust the antenna a little as you will have more sig getting to the antenna there for swr will go up and you need to adjust the antenna .

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PostSubject: Re: coax problems   coax problems Icon_minitimeFri Jun 26, 2020 3:06 pm

More signals getting to the antenna won't put the swr up Stephen. The swr is adjusted as to which frequency you are on. Co-ax shouldn't move your swr either unless there is a problem with it.

Paul.
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PostSubject: Re: coax problems   coax problems Icon_minitimeFri Jun 26, 2020 5:53 pm

hi all,thanks for your input.im on a learning curve at the moment so if I question anything its because I need to know the hows and whys.first I think the rf choke will only work if I have rf problems and the lenth of coax will just be the same so not change swr
I changed rg213,as Mitch said because 25m of rg58 has a 30% loss

spoke to Knights today and they said lenth of coax should not change swr
intresting point by pagan about 213 v rg8 but still the point why as swr gone up when better coax gives less noise and loss
iagree with sgt bilko its not more signal or you would have to re-swr when using an amp
juat checked swr on mid ch 1 1.5 ch 40 1.2 with matchers ch 1 3.2 ch 402.5 without
I know I raise more Q then A but am greatfull for your replies
73s mark
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PostSubject: Re: coax problems   coax problems Icon_minitimeFri Jun 26, 2020 7:09 pm

hi all just a quick update checked coax for continuity and the meter buzzes when center pin and outer are touched with probes, ill have to check if antenna or just new coax has the short, just trying to get wife up the aluminium ladder in this thunder storm lol
thanks again
73s mark
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PostSubject: Re: coax problems   coax problems Icon_minitimeFri Jun 26, 2020 8:38 pm

The silver rod is DC short internally, it's the matching coil inside the aerial base, if you want to check the coax you'll have to disconnect the aerial first, unfortunately. Can you beg/borrow or otherwise get the use of an aerial analyzer? I set mine up with an MFJ-269C a few years ago, now sold on (hardly used it but got my money back for it). From what you describe about swr it appears the aerial's too short. Can you check higher up frequencies? 10m band? Just to give you an idea.
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PostSubject: Re: coax problems   coax problems Icon_minitimeSat Jun 27, 2020 1:19 pm

thanks mitch your a star.coax disconnected and ok.did not know silver rod has internal short.put rg 58 back on and swr back down to 1;2
so think I will leave it like this and when a new antenna comes swr it all together on the ground
back up to 40watts so makes up for the loss with 25m of rg58
thanks again for those who tried to help.CT group are a good bunch
cheers mark
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PostSubject: Re: coax problems   coax problems Icon_minitimeSat Jun 27, 2020 2:22 pm

Markone wrote:
hi all hope your well
need a little help,just up graded 25m of rg58 for 10m of rg213 and swr gone from 1.2 upto 2.2.
antenna is a silver rod set up as instructed and 1.2 out of the box great,put it on a pole on gable end and 1.2,great.thought id replace the coax with 213 ready for a better antenna in coming months but swr now 2.2
now got this down to 1.2 with zetagi matchers but watts on meter now dropped from 40w to 25w,what the hell is happening,the 213 was new from knight with plugs fitted.
heard about coax being different lenths but no probs when first set up
read about coax being multiples of 11 feet,10 metres is about that.hope ive made sence
regards mark

So you're not going to like the answer to this. Because of the losses there are in RG58 it was masking an issue with your antenna, the fact you have no or a very poor RF ground.

Now you've heard about coax having to be a certain length to get low SWR, whilst it works it's actually an old wives tale that got traction because people didn't understand why it works and why if it does it's showing you you've got a problem. If your coax is being used by the antenna system to try to compensate for a poor RF ground then altering the length of your coax will affect the tuning of the antenna. THIS IS NOT A GOOD THING because it also means you're also suffering issues from a thing called Common Mode RFI. CM RFI was the reason that CBers caused TVI to neighbours and on computer speakers because it isn't just your antenna your signal is radiating from, it's your coax too. You get more noise on receive, you get distorted audio when you transmit.

Ultimately unless you're willing to put some radials on the antenna and install a RF choke near the antenna you're going to have to live with it. You could change to RG213 with it as it is but because the coax is being used for a RF ground you'll have to alter the length to get a low SWR.
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PostSubject: Re: coax problems   coax problems Icon_minitimeSat Jun 27, 2020 2:49 pm

thanks for the reply,when I set up the antenna to the factory marks swr 1;2 in the garden.put it on side of house swr 1;2 only changed when rg213 was used.dont have to much noise s4 normally and audio is very good.qso with 21 countries and over 50 staions,tried rf choke and made no difference at all with rg58 not sure if I suffer from rfi.is there way to tell apart from above.if rfi is the problem I must have been lucky to get swr 1;2 with factory marks.if I suffer from rfi how do I stop it, radials not an option,rf choke did nothing
regards mark



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PostSubject: Re: coax problems   coax problems Icon_minitimeSat Jun 27, 2020 9:48 pm

Markone wrote:
... when I set up the antenna to the factory marks ...

I think that was your first mistake. You should've checked the SWR on the ground with a short patch lead before install, I did mine with an analyzer when it was on a 5ft pole, before mounting it on the garage (on a 15ft pole) it's not perfect but it gets you in the ball park range.

So now you're reading 40 watts instead of 25, but it's likely there's 25 or less being radiated. Dare you trim the RG58 to the right length? The less coax the better tbh, especially with RG58, just use what you need, but there's always the chance it'll be the same as the 213 though. Like Northern Crusader says, you have a bit of a problem, but don't give up hope, you'll get there in the end.

Just look at it as a learning experience and gaining valuable insight into the dark arts of antennas, practice make perfect Very Happy

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PostSubject: Re: coax problems   coax problems Icon_minitimeSun Jun 28, 2020 9:12 am

As said the lower swr is down to the extra loss in the rg58.
And as Mitch say's tune your antenna with a short patch cable before you put it up in the air.
A lot of good advice in this post/topic for others to take in as well.
Your antenna will be the same length on a 3' pole as on a 30' pole only the coax will alter swr with a false reading sort of.


Last edited by Alan Pilot on Sun Jun 28, 2020 11:14 am; edited 1 time in total
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PostSubject: Re: coax problems   coax problems Icon_minitimeSun Jun 28, 2020 9:52 am

Markone wrote:
thanks for the reply,when I set up the antenna to the factory marks swr 1;2 in the garden.put it on side of house swr 1;2 only changed when rg213 was used.dont have to much noise s4 normally and audio is very good.qso with 21 countries and over 50 staions,tried rf choke and made no difference at all with rg58 not sure if I suffer from rfi.is there way to tell apart from above.if rfi is the problem I must have been lucky to get swr 1;2 with factory marks.if I suffer from rfi how do I stop it, radials not an option,rf choke did nothing
regards mark

How were you measuring the SWR to get the 1.2?  Was it through the old RG58 coax still?

My suspicion would be the same as others - the old coax had a fair amount of loss, and if you didn't have a perfect impedance match, whether caused by poor ground or antenna issue, that higher loss is masking a poor SWR as you're losing a lot of the return signal used as part of the SWR calculation.

My advice would be to get the matcher and any amp out of circuit, tune the antenna properly for lowest SWR on whatever frequency/channel you prefer, and then see where you are.  For a single band antenna, you don't need a matcher/tuner unless you have another problem.
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PostSubject: Re: coax problems   coax problems Icon_minitimeSun Jun 28, 2020 2:33 pm

thanks to all who replied and tried to help
I think most off you agree its a common mode rfi problem,masked by the loss of 25m of rg58
if the case and I cut down the rg213 to get the swr down am I still masking an rfi problem because coax length should not matter
and if so how do I stop rfi problem, can make a choke but didn't know end fed 1/2 waves needed a ground
if ground needed how do achieve this
like I stated before I like to know the how's and whys
thanks for your patient's
regards mark
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PostSubject: Re: coax problems   coax problems Icon_minitimeSun Jun 28, 2020 3:58 pm

To do it properly you'd need to make radials and attach them to the antenna bracket that bolts onto the pole. Ideally you'd have four of them roughly about 9ft long. If you look at an antenna like the Sirio Tornado 27, something like that although depending on what options were available you could use wire.

The alternative is to accept you can't do that and DELIBERATELY use some of the coax instead. You'd do that by using a RF choke on the coax not at the antenna feedpoint but so far back from it, somewhere between 6-8ft from it (this is because of a thing called the "velocity factor" or VF of the coax which means you reduce the length from the 9ft quarter wave due to the VF and the fact it makes a conductor appear electrically longer ). That would mean you had common mode RFI in the 6-8ft of coax between the choke and the antenna as the antenna would be using it for a ground, but the choke would stop it getting back to your radio.
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PostSubject: Re: coax problems   coax problems Icon_minitimeSun Jun 28, 2020 4:35 pm

I've not got any ground planes on my silver rod, Mark, and it tunes fine, I have ferrite beads for a choke/balun but it's not very high and there's a load of RG58 so mine is a very poor system, lol. I must be losing half my signal at least, but I'll sort it soon, promise.

I can only think of a couple of ways you could fix it. One is to take the aerial down and set it up on the ground (up a bit helps) with a shorter lead, ignoring the factory setting marks and getting more accurate measurements, and tune it to band centre. The problem there though is getting your kit outdoors to do it, power supply, radio and SWR meter etc. if you can, that's why I used an analyzer, but I did have one at the time and it was shame not to use it. If you can borrow one it'll make it easier for sure. Are you in a radio club? They usually have them if you ask nicely.

Another, maybe simpler way if you can get to the aerial sections, is to put the RG213 back on and loosen, then slide one of the aerial sections (try down first) and keep checking the SWR until it comes down enough, e.g. 1.2:1 band centre. If you get it tuned you should have less common mode rfi as well so win win.

On a slightly different note, do you have a dummy load and power meter? If so measure the radio output into the dummy load with a short lead, note power, then swap the short lead for the 25m of RG58 and note the reading again (and go "eek"), you might be surprised how lossy it is. It gets worse the higher the frequency, for VHF and UHF it's near useless, I get perfect 1:1 at 430Mhz on 20ft of open ended (nothing connected) RG58, it absorbs the lot, lol.
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PostSubject: Re: coax problems   coax problems Icon_minitimeSun Jun 28, 2020 5:37 pm

ok thanks mitch and northern crusader,if im having to take antenna down to tune might as well buy a good one (recommendations) and start again.do it right do it once with rg213 cut to minimum lenth
don't want radials but can easily do a choke 8 foot in line,that wood put choke in side the house,
thinking about sirio gain master (no radials necessary)5/8 wave or imax 2000.
any comments on this idea
73s mark
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PostSubject: Re: coax problems   coax problems Icon_minitimeSun Jun 28, 2020 5:57 pm

Ha ha, er yes, sorry, lol. I had an imax 2000 mounted on the house a few years ago. When the wind blew, good lord, it sounded like someone was rolling beer barrels across the roof, it used to wake us up at night, that's how loud it was, the noise was horrendous, so couldn't recomment one unless you're deaf. And it bent over almost double too, and it was only about 10ft off the ground (I'm in a bungalow).

On the other hand a friend of mine has recently bought a .625 (5/8ths?) Gainmaster and he says it's the best aerial he's ever had, he's well happy with his, so if you have the cash it could be the one for you too. I've never had one myself but others have said they're good as well. They don't have ground planes and they do have a built in choke apparently.
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PostSubject: Re: coax problems   coax problems Icon_minitimeSun Jun 28, 2020 6:04 pm

hi mitch would starting again with the gain master seem a plan
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PostSubject: Re: coax problems   coax problems Icon_minitimeSun Jun 28, 2020 6:18 pm

Seems like a good idea to me Very Happy

(Just don't use RG58 this time)
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PostSubject: Re: coax problems   coax problems Icon_minitimeSun Jun 28, 2020 6:23 pm

thanks for the help
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PostSubject: Re: coax problems   coax problems Icon_minitimeMon Jun 29, 2020 12:18 pm

Sgt Bilko wrote:
More signals getting to the antenna won't put the swr up Stephen. The swr is adjusted as to which frequency you are on. Co-ax shouldn't move your swr either unless there is a problem with it.

Paul.
OOPPSs Yes you are right I meant more power getting to the antenna just adjust the antenna to bring swr down,
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PostSubject: Re: coax problems   coax problems Icon_minitimeMon Jun 29, 2020 2:57 pm

i have a gainmaster 5/8 and its been up about 9 years even in the storms last year
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PostSubject: Re: coax problems   coax problems Icon_minitimeTue Jun 30, 2020 4:12 am

Markone wrote:

don't want radials but can easily do a choke 8 foot in line,that wood put choke in side the house,
thinking about sirio gain master (no radials necessary)5/8 wave or imax 2000.
any comments on this idea
73s mark

Imax 2000 needs radials too, have a read of the following which explains why it and the A99 does and why all end feds do too if you want a bit of understanding.

https://www.w8ji.com/end-fed_vertical.htm#IMAX

You can buy a proper ground plane kit for the Imax but it's four 6ft long radials. Sirio Gainmaster doesn't need them because although it's got an antenna socket on one end of it the feedpoint of the antenna is actually in the middle of it, it's basically like a vertical dipole in how it works. It also shouldn't need a RF choke either because the red coax coil at the base of the Gainmaster acts as one.

Given you don't want radials and for the sake of simplicity then to get the best performance I think the Gainmaster might be one worth considering for you.

I'd still keep the silver rod, you can always chuck it in the car, go out on some hills and tie it to a fence post and use it for portable work.
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