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Author Topic: RF in house?  (Read 2075 times)
CHRISDX
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Posts: 244




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« on: October 10, 2012, 06:46:12 PM »

Hey all, I meant to ask this for a week now.  When on 10m and the antron vertical mounted on a chimney mast with the base approx 20' high, and I tune up on low power to a 1:1 SWR, then go to full 100w power, my computer monitor gets heavy interference and even shuts down.  

When I modulate itr comes out the nearby surround subwoofer speaker. I notice when tuning up that as I dial the match in, the interference gets much worse. Shouldnt it be the other way?

This tells me the RF is not all going out to the antenna where it should be.  The antenna has RG58 down to the alpha delta switch, then a short patch to the Icom.

Not sure where to begin because the radio and tuner show a good match so shouldnt the RF be staying outside? Something doesnt seem right though.

I do have an MFJ 259B antenna analyzer but not sure what to look for...  (and I do realize *maybe* there is really no problem, I can contact my fellow hams on 10m - not really quite what it should be but its ball park with the gang, sluightly subpar and that could be my locale.)
« Last Edit: October 10, 2012, 06:51:59 PM by CHRISDX » Logged
VA7CPC
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« Reply #1 on: October 10, 2012, 06:52:18 PM »

SWR has nothing to do with radiation efficiency, or RFI level.

. . . How far is your antenna from your shack?  You might just be in the "near field" of the antenna -- getting RFI from its radiation.

I don't know your antenna, but with any vertical there are two questions:

. . . Do you have a balun at the antenna feedpoint?   If not, I'd start with a 1:1 current balun.

. . . Do you have an RF ground system -- radials, or a groundplane?  If not, the coax shield is carrying
. . . a lot of RF, right down to your rig. 

           Charles
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CHRISDX
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Posts: 244




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« Reply #2 on: October 10, 2012, 07:08:31 PM »

Charles, the chimney the mast is on is right behind me. About a 25' coax comes right off the bottom of the vertical to my switch. (I disconnect it and toss out the window when it storms).

There is no ground plane on the antenna although they do sell one and there is no room for a ground radial system. Thsi house is 3' from the property line and the chimney is halfway down that side.

I like the current balun idea the descriptions mention stopping RF coming back down, perfect. And have a question, this product says it runs the feedline through ferrite beads:

http://www.hamradio.com/detail.cfm?pid=H0-002260
The W2DU Maxi-Baluns owe their extremely high power capability to the fact that they contain no ferrite cores to saturate. Even at high SWR levels, the W2DU Baluns won't generate harmonic radiation. The internal feed line passes through ferrite beads which prevent RF current from flowing on the coax shield.

So I do have a box of high grade ferrite snap togethers, would those do me any good or is there some engineering within the commercial product above?

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W5DXP
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« Reply #3 on: October 10, 2012, 07:45:35 PM »

There is no ground plane on the antenna ...

What does the antenna use for a counterpoise, i.e. to what is the braid of the coax connected at the antenna feedpoint?
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
CHRISDX
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Posts: 244




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« Reply #4 on: October 10, 2012, 07:58:10 PM »

There is an SO-239 on the bottom of the 10M stick.  My coax goes from this straight down in the window to my switch then tuner.

I am reading up now on Baluns which as Charles said, SWR can be fine at the radio and still have major RF coming off the coax feedline all the way down to the rig which I am now sure is my issue.

I am wondering if a 1:1 current balun up at the antenna feedpoint will just stop RF coming back down, or if in fact I would also gain performance in radiation efficiency (more RF going out the vertical). Either way this looks like what I need.

I had thought that a balun was just to match a balanced to unbalanced not for this other function.

Here is some info I'm looking over:
http://www.g0ksc.co.uk/creatingabalun.html

There is no ground plane on the antenna ...

What does the antenna use for a counterpoise, i.e. to what is the braid of the coax connected at the antenna feedpoint?
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G8JNJ
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« Reply #5 on: October 11, 2012, 12:58:49 AM »

Hi,

The Antron is an end-fed 1/2 wave antenna.

Despite what the manufacturers may say, it still requires some sort of groundplane to work correctly.

In most cases this is provided by by the outer screen of the coax that is feeding it.

If you fit a choke balun to the coax you remove the groundplane and the antenna will work less effectively. This may indirectly solve your RF in the shack problem. But it's not going to help you radiate the best signal.

Here's my suggestions - apply them in this order:-

1. Add an even number of 1/4 wave long (about 8ft) radial wires to the base of the Antron. They can drape across the roof, or down the side of the house it doesn't matter. These will help improve radiation efficiency and partially decouple RF from the outer of the coax.

2. Add a good quality 1:1 choking balun (see the G3TXQ link further down this note) to the coax at the antenna, but below the point where the radial wires are attached. The choke needs to be between the radio and the radials. Not between the radials and the antenna. This will further decouple RF from the braid of the coax.

3.  It is likely that you will still have RFI problems due the proximity of you antenna to the shack. If you can't increase the distance between antenna and shack. Either horizontally, or vertically by attaching it to a longer support pole. Then you will have to deal with the problem of RF getting into equipment in your shack.  Ferrite beads are the answer here.

Take a look at these links

http://www.eham.net/ehamforum/smf/index.php/topic,83193.0.html

http://www.karinya.net/g3txq/chokes/

http://webzoom.freewebs.com/g8jnj/EMC%20and%20Amateur%20radio.pdf

Regards,

Martin - G8JNJ

www.g8jnj.webs.com
« Last Edit: October 11, 2012, 01:01:39 AM by G8JNJ » Logged
W5DXP
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« Reply #6 on: October 11, 2012, 04:53:00 AM »

The Antron is an end-fed 1/2 wave antenna.

Is it a "sleeve" dipole or end-fed monopole? To what is the coax braid attached at the antenna feedpoint?

If there is no counterpoise (except for the coax braid), a 1:1 choke is merely a band-aid that doesn't come close to solving the basic problem.

Some CB antennas are well-designed and some are absolute junk.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
AA4PB
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Posts: 12696




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« Reply #7 on: October 11, 2012, 05:16:38 AM »

If its and end-fed 1/2 wave then, just like the PAR end-feds, it uses the coax shield as the counterpoise. That can bring RF back down into the shack. If you add a balun or choke to stop the RF from flowing on the coax shield then you have effectivly disconnected the counterpoise and the antenna won't work very well and will probably be difficult to load.

I expect the RFI increases as you adjust the tuner for minimum SWR because the transmitter fold-back circuit allows more RF output as the SWR is lowered. More RF delivered to the antenna means more RF comming back into the shack.

If you add four 1/4 wave long radials to the base of the antenna, connected to the coax shield you will minimize the RF flowing back down the coax and probably improve the antenna performance as well.
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WA3SKN
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« Reply #8 on: October 11, 2012, 11:10:35 AM »

Sounds like the antenna is doing what it is supposed to do... radiate a signal!
However, the subwoofer is acting like a receiver... not what it is supposed to do.  And it sounds like your antenna is fairly close to the eqpt.
So the answer(s) are distance, shielding, and filtering.  How far apart can you get the antenna and subwoofer?  The more space the better.  Can you get some shielding between them?  You can do a lot with aluminum foil or screening!  The comes filtering.  You must determine how the signals are getting into the eqpt and are being detected, then you can filter effectively.  But doen't blame the antenna... it is doing it's job!
73s.

-Mike.
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KD0REQ
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Posts: 858




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« Reply #9 on: October 11, 2012, 02:53:52 PM »

the monitor, subwoofer, et al are in compliance with the FCC labelling that they "must accept any radio interference presented to them."  that's squirrelly language which doesn't say they have to work right when in a strong RF field.

the antenna is working fine, putting that field out.

obviously, the radio tunes, so it's OK.

the problem is that RF field is not supposed to be that close to the stuff.  all the suggestions prior to this are correct... start with grounding the radio to a good earth ground, and then adding baluns, counterpoises, and ferrites in that order.  the general idea is that RF should all be going up and out, and you have a bit much coming down and in yet.  once that issue is solved, block what's left in the badly-designed, shoddily-assembled, and vulnerable consumer electronics with ferrites and maybe extra shielding on the cables.
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G0GQK
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« Reply #10 on: October 12, 2012, 12:32:52 PM »

Try winding half a dozen coils in the coax about a foot radius, that should cure it

Mel G0GQK
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VE3FMC
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« Reply #11 on: October 12, 2012, 12:47:08 PM »

Try winding half a dozen coils in the coax about a foot radius, that should cure it

Mel G0GQK

That is exactly what he needs to do. Been there and done that with that antenna.
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VA7CPC
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« Reply #12 on: October 12, 2012, 01:30:04 PM »

+1 to G8JNJ's suggestions -- all good, and in a logical order.

Ferrites on _every_ wire -- power and data -- leading to the computer system would also be a good idea.  That cures a lot of problems.

Yes, the ferrites in your W2DU balun kit are split ferrites (I think!), and you can use them anywhere.

When I was installing an HF radio on my boat, I ordered $100 worth of ferrites from Amidon.  They're mostly all used, now, between the boat and the home shack.  It was one of the best ham-radio "investments" I ever made.

.            Charles
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