Rf choke and swr readings

Homebase antenna advice and troubleshooting

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ajrm
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Joined: Thu Jun 11, 2015 7:18 am

Tue Mar 01, 2016 10:36 pm

Coming back to CB radio after 40 odd years, and my brain is trying to remember stuff long forgotten. The last base setup I had was a 40 channel Cobra running through a dv27 mounted on a fire guard in the front room, and I got fantastic DX contacts from all over the world.
So, I thought you lot could help!
I have a CRE 8900 radio set up for UK CB channels ( am FM and SSB) and a Thunderpole 5. We live in a very old house, quite remote ( nearest road is 1/4 mile away, nearest neighbour, 400 metres away) in a clearing in the the middle of a wood about 50ft above sea level.
I have the Thunderpole on a 3 meter steel pole, the base of the antenna is about 5 metres above the ground and above and to the side of a large steel shed. I can't physically get it any higher at the moment due to a very steep roof and high walls, there is about 20 metres of coax feeding the antenna.

The problem I have is a very high swr, lowest is 3:1.

I have used the radio on a mag mount mobile antenna and swr is fine ( around 1.3:1 max) so I know the radio is good.

I read about RF chokes that may reduce the swr a little, so I fitted one about 16" below the base of the antenna. It consists of 8 loops of coax with 8" diameter loops.

Now the swr is around 1.1:1 and raises to about 1.5:1 at 1 MHz either side of the lowest reading.

So, my question. Is the rf choke really reducing the swr to such a low level, or am I getting a false reading?
I am not at home at the moment and I haven't tried a radio check yet ( not heard anyone local) but the set seems to pick up well, I have heard SSB and am transmissions from all over Europe and USA.

Dragonfly
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Wed Mar 02, 2016 10:05 am

I would think the high SWR will have been caused by a shorted PL plug on the patch lead or the main coax , RF choke should have little effect on the SWR .

ajrm
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Wed Mar 02, 2016 5:51 pm

I thought that too, but I have been fitting plugs on coax for years and I always check with a meter that there are no shorts, and in the case of this piece of coax and pl259's, the connections were perfect.
I suppose I could take the choke out and check again, but, that's a lot of work!
I will be home in a couple of weeks so will try to find someone to talk to, I have heard a couple of people talking on ch19 a few times and they seem pretty close, in fact, I know of one of them and he lives not too far away.

Dragonfly
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Fri Mar 04, 2016 2:25 pm

Is the antenna new or second hand , It should be 18ft long according to Thunderpoll specs but if someone has cut a couple of foot of the top steel whip that would put it out of tune .
http://www.thunderpole.co.uk/base-cb-an ... ole-5.html

Dragonfly
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Fri Mar 04, 2016 2:37 pm

Found on the net that there is a cutting chart for the Thunderpoll 5 .
Attachments
cutting chart.jpg
Thunderpoll 5 cutting chart
cutting chart.jpg (51.12 KiB) Viewed 2485 times

ajrm
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Mon Mar 07, 2016 4:30 pm

It's a new Thunderpole 5 and I used the cutting chart to cut the whip at 32".
My radio has the ability to transmit from 25.615 to 30.105, so 27-28 MHz right where I want the best swr anyway.
Still don't understand why the swr is so low with the RF choke fitted though.

Dragonfly
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Tue Mar 08, 2016 8:25 pm

I Recon RF must be coming back down the coax outer braid causing an imballance of the ground length and the Ugly Balun will be stopping the RF coming down . I notice the Gainmaster uses a Balun as standard so they probably have the same problem without one .

ajrm
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Tue Mar 15, 2016 2:49 pm

Well, I managed to sort it all out and it turns out it was user error!
I just discovered that instead of my roll of RG8 mini, I used a roll of RG6.
So, I replaced it with RG8, took off the RF choke and swr is now down to between 1.1:1 to 1.3:1 and tx power is up to where it should be.
Thanks to all who helped, sorry for being so stupid!
I did say it was a long time since I used a CB radio...........

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