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Big Cheese

Registered: 11-2007
Location: Wiltshire, UK
Posts: 2655
Karma: 12 (+12/-0)
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RF feedback? i'm not sure?


right... when i switch the burner on at home with a power mic connected i get a carrier on ssb, if i turn the mic right down(nearly off)it disapears. If i use a standard mic this doesnt happen. what could be the problem?
the antenna is in the loft above the room i have the radio in and the coax runs along the loft floor for about 5 foot(horizontal)then out through the hatch down to the room. I have had a simular problem when tryin out a radio with a magmount in the same room. If anyone can give me a list of things to try i would be gratefull, i've read somewhere it might be cos there is not enough groundplane on the antenna or bad earths or summit, help would be apreciated, or a check list i can start doing to narrow it down. i have read a bit on the internet but get a bit lost with words like counterpoise???? help

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26FB118
6/Feb/2008, 10:43 pm Link to this post Send Email to topbuzzuk   Send PM to topbuzzuk Yahoo
 
yamanx1 Profile
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Head Honcho

Registered: 09-2007
Location: In a world of my own
Posts: 4894
Karma: 40 (+41/-1)
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Re: RF feedback? i'm not sure?


Sounds like it could be a few things.

I would, check the mic wiring, particularly make sure no possible contact between pins.

Try with head phones.

The power mic will cause the linear to actually put out a higher average pep, increasing your power up that coax, so your assumption about rf feedback could be right, coming from your coax, a method of eliminating this, without changing all your coax to expensive RG213 is an rf choke.

What you need to do is coil around 8-10 feet (for 11m only for all other bands as well you'll need 18-21 feet)of coax around a plastic pipe. Making sure all the coil touch but the last one doesnt touch the first. Normally you would place this near the mast as a balun (Balances unbalanced coax with balanced aerial) but in your case I would do it straight out the back of the radio.

More information here RF Choke/Balun


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By all means think yourself big, but never think others small.... R.F.Scott
7/Feb/2008, 11:29 am Link to this post Send Email to yamanx1   Send PM to yamanx1
 
topbuzzuk Profile
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Big Cheese

Registered: 11-2007
Location: Wiltshire, UK
Posts: 2655
Karma: 12 (+12/-0)
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Re: RF feedback? i'm not sure?


all sorted thanks to the balun, ive got rid of the carrier on ssb with the mic on full and the burner going. i havnt had any on air reports as yet but i think its done, thanks Dave

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26FB118
9/Feb/2008, 12:05 pm Link to this post Send Email to topbuzzuk   Send PM to topbuzzuk Yahoo
 
topbuzzuk Profile
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Big Cheese

Registered: 11-2007
Location: Wiltshire, UK
Posts: 2655
Karma: 12 (+12/-0)
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Re: RF feedback? i'm not sure?


well i just put the astatic in the discovery on the ranger and i had the same problem as i did in the homebase, i have cured the problem but have never heard of this before. i wound the mic cable(plug end)twice around a ferrite choke and hey presto no more carrier on ssb with the power mic. i wonder if that may have worked on the homebase???

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26FB118
24/Feb/2008, 8:05 pm Link to this post Send Email to topbuzzuk   Send PM to topbuzzuk Yahoo
 
John193 Profile
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Freebander

Registered: 11-2008
Location: South Glos
Posts: 260
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Re: RF feedback? i'm not sure?


Was that a balun or rf choke? sounded cunningly like an rf coaxial choke. sounds also like the linear is generating a bit of sheild current on the coax, the fix you've tried is undoubtedly the cheapest fix for that, a copper sleeve over the coax then linked to the sheild at antenna does the same thing too if your short on coax length. John193
27/Mar/2009, 2:41 pm Link to this post Send Email to John193   Send PM to John193 MSN
 
Hot Fusion Profile
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Management

Registered: 03-2008
Location: South West England
Posts: 1389
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Re: RF feedback? i'm not sure?


Glad you got the problem sorted - can be a tricky one to solve.

For the sake of general information, I'll make a point about identifying RF feedback on the audio, as amplified microphones will pick up fairly quiet sounds which you may be unaware of, such as the noise of a fan somewhere, leading to the 'carrier' on SSB whilst not speaking.

Slowly increase the mic gain, whilst making little clicking noises - just enough to create a deflection, and as you reach RF feedback threshold the observed output will suddenly leap up and sustain - this indicates an RF feedback problem.

Ordinary background noise will simply increase in relation to the mic gain.

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Andy - 26 FB107
28/Mar/2009, 8:58 am Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 
topbuzzuk Profile
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Big Cheese

Registered: 11-2007
Location: Wiltshire, UK
Posts: 2655
Karma: 12 (+12/-0)
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Re: RF feedback? i'm not sure?


hi john, it was a choke/balun i wrapped about 9 foot of coax around a 4 inch plastic pipe and fitted it to the antenna end of coax, worked a treat.. the mobile mic issue was sorted with a ferrite on the mic lead

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26FB118
28/Mar/2009, 9:24 pm Link to this post Send Email to topbuzzuk   Send PM to topbuzzuk Yahoo
 


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