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Author Topic: RF in Shack ??  (Read 4063 times)
KJ6TSX
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Posts: 116




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« on: May 27, 2012, 05:25:03 PM »

I put my new parallel antenna up and I am having trouble with getting the antenna tuned. or some reason on 17 meters i get so much rf in the shack that when I transmit my Radio speaker, computer speakers all receive my transmission, was so bad I couldn't complete my communication.

So here is my setup: Parallel dipole inverted V 37 feet  high, coax comes to shack ten wraps of coax in circle about 12 inches in diameter 6 feet to rig
MFJ Tuner,

Observation: 80 meters tunes, 40 meters must use bypass mode, 20 meters tunes, 17, 15 meter tunes but RF In shack, 12, 10 meter tunes

Could use some advice Just not sure were to start, did I do the choke wrong?

Thanks
George
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WB6BYU
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« Reply #1 on: May 27, 2012, 06:42:30 PM »

G3TXQ has a lot of information on coax chokes here:
http://www.karinya.net/g3txq/chokes/

I suspect that the choke is too large to be effective on the higher bands.

Is the choke at the antenna or in the shack?


The important question is, what is the SWR without the tuner on each band?
The fact that you can tune it doesn't give much information, but knowing how
well the antenna matches is a big help in diagnosing the problem.

The fact that you have to use bypass on mode on 40m is suspicious:  most
tuners can match a 50 ohm load, so if the SWR is low the tuner should still
be able to match it.  (Some L-network tuners will be the exception.)
What model antenna tuner are you using?  Does it have a switched inductor?
If so, you may need to check the switch contacts for arcing or other signs
that they might not be making good contact in the 40m position.

What is the angle between the wires?  Is the coax close to one wire than the
other where it leaves the antenna?  How long is your coax?  RF in the shack
can be a function of the coax length - try adding 10' or so of coax and see
if that changes the symptoms.
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KJ6TSX
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Posts: 116




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« Reply #2 on: May 27, 2012, 08:16:59 PM »

Thanks for Helping
I checked out that web page I understand the concept but an not sure how  to apply it

Quote
I suspect that the choke is too large to be effective on the higher bands.
From the looks of the chart You may be right (13 turns 12 inch diameter)

Quote
Is the choke at the antenna or in the shack?
The choke is in the shack (about 8 feet from radio)

Total length from radio to antenna 102 feet, coax total length 125 feet RG213
elements are at 90 degree to each other, coax is 45 degrees to each element

The following is what I was able to tune, I got 40 meters to tune (Just had to work a little more Grin)

Frequency ----- SWR no tuner ----- SWR tuner
80                        1:1.5                 1:1
40                        1:1.2                 1:1
20                        1:1.3                 1:1
17                        1:1.5                 1:1
15                        1:1.5                 1:1
12                        1:1.5                 1:1.2
10                        1:1.2                 1:1
Looking at this none of these are bad at all
I have a piece of 4 1/4 plastic how many turns do you think?  11??

Thanks for all the help
George
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AA5WG
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Posts: 496




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« Reply #3 on: May 27, 2012, 08:41:43 PM »

George:

Update:

Check to see if you are using a single point ground system.  i.e. "...ground each piece of equipment independently to a single central point", thus, a single point ground system.
Here is a link about grounding.  Look at "The Cure" section more than once.

http://www.radioworks.com/nbgnd.html#Cure

How long is your antenna?  

Ideally the length from one end of your antenna to your antenna coupler should not be a multiple of a half wavelength at the operating frequency.

Ideally, use coax for one band only.

Ideally, use balanced line, ladder line, for multi-band use.

Ideally, use a mechanically and electrically balanced antenna coupler for multi-band use.

The link antenna coupler is such a device.

I did not realize you were using a fan dipole.  My above suggestions were for a single antenna with open wire feeders.  Still, try checking your ground system.  Try not grounding your antenna coupler and then with ground to see if there is a change.   

Chuck

« Last Edit: May 28, 2012, 09:21:20 AM by AA5WG » Logged
WB6BYU
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Posts: 13237




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« Reply #4 on: May 28, 2012, 08:07:12 AM »

If you can get those SWRs expressed the right way around (1.5 : 1) then you don't need a tuner.

Try the 11 turns on the 4" form, at least that is optimized for 17m.  You can see how much
difference it makes.  The choke will be most effective right at the antenna:  if you have
enough spare cable you can put a second one in the shack.  Don't be surprised if the SWR
changes on some bands, however.

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WX7G
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« Reply #5 on: May 28, 2012, 08:11:46 AM »

Ugly baluns are really a one band affair. For your all band antenna I would try NO balun first. If you still have RF in the shack a 1:1 ferrite core balun mounted at the antenna might help. Another thing to try - and you might try this first - is 13.0' radial wire connected to the GND post on the tuner or the transceiver.
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W5DXP
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« Reply #6 on: May 28, 2012, 08:46:03 AM »

Parallel dipole inverted V 37 feet  high, ...

Is a "parallel dipole" the same thing as a fan dipole?
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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.
WB6BYU
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Posts: 13237




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« Reply #7 on: May 28, 2012, 08:53:14 AM »

It's not a TRUE "fan dipole" in the original sense, where the antenna would have a reasonably low
SWR across the whole HF range like one of these:

http://ascsignal.com/files/hf/omin_directional/1765.pdf


But "parallel dipole" would be multiple dipoles on a common feedpoint that aren't fanned out.

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KJ6TSX
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Posts: 116




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« Reply #8 on: May 28, 2012, 09:47:33 AM »

Here's a picture of the Parallel dipole I installed, http://www.alphadeltacom.com/antenna-photos.html I believe it would also be called a fan dipole. But defiantly not like the one in the WB6BYU link ( wish I could though)

Tried completely removing ground from rig (no luck)
Tried undoing my coax wire coil (no Luck)

Going to make a ugly balun and put a second ground rod in closer to the shack. we will see what happens
George

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WB6BYU
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« Reply #9 on: May 28, 2012, 10:26:47 AM »

Do try adding a quarter wave wire radial (about 13') to the tuner and/or the rig (try both) and tuck
it around the edge of the shack and/or under the carpet.  That may reduce the RF problems on 17m.
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KJ6TSX
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Posts: 116




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« Reply #10 on: May 28, 2012, 10:51:35 AM »


Tried the 11 turn balun no luck

Tried counter poise 13 feet long no luck

I am off to buy a second ground rod and some grounding wire

Thanks
George
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KJ6TSX
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Posts: 116




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« Reply #11 on: May 28, 2012, 04:24:28 PM »

Well I don't know exactly what the problem was but by properly grounding the shack independently of the antenna, the rf in the speakers was almost gone. I moved the speakers and now it's all gone.
I think I know why my tuner is having so much trouble tuning the antenna. It is trying to tune 3 antennas at once. Why am I worried about tuning below 1.5:1 ?? My radio will only put out 50 watts at 1.5:1 but at 1:1 it will put out 120 watts. Doesn't seem quite right but that's what happens
Thanks Everyone for all the help
73s
George
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KC4MOP
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Posts: 733




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« Reply #12 on: May 31, 2012, 03:51:18 AM »

Well I don't know exactly what the problem was but by properly grounding the shack independently of the antenna, the rf in the speakers was almost gone. I moved the speakers and now it's all gone.
I think I know why my tuner is having so much trouble tuning the antenna. It is trying to tune 3 antennas at once. Why am I worried about tuning below 1.5:1 ?? My radio will only put out 50 watts at 1.5:1 but at 1:1 it will put out 120 watts. Doesn't seem quite right but that's what happens
Thanks Everyone for all the help
73s
George
This parallel dipole is tuned for some band? You mention 3 antennas. Are any of these wires cut to be resonant on any band?
The parallel dipole might be the problem for 17M. The tuner will try to make a 50 ohm match to this antenna system but if each wire is trying to accept 17M energy then that may be the cause of RFI.
The foldback circuits in your radio are looking at more than the SWR. There may be excessive reactance, either inductive or capacitive from the antenna at certain frequencies that are causing the radio to cut back power.
Do you own an MFJ 259? That will tell the whole story. You will see the type of reactance that is giving your radio trouble. Attach the MFJ259 to the input of the tuner and try to get the X value below 20.
A lot of transceivers go bonkers on 40M and start folding back power for the slightest SWR.
Fred
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KC7YE
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« Reply #13 on: May 31, 2012, 06:36:10 AM »

With my poor set up in condo had lots of RF in shack on all bands. Cured by running ground braid (from flea market & DX engineering, others) to water line about 35 ft. Braid under rugs and along wall. Bingo ! RF gone. XYL not happy.
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W5DXP
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« Reply #14 on: May 31, 2012, 09:37:49 AM »

Cured by running ground braid (from flea market & DX engineering, others) to water line about 35 ft.

Please understand that on the frequencies that the 35' "ground" braid is an odd multiple of 1/4WL, it is ground only on the end that is grounded. Smiley On the opposite end, it is virtually an open circuit.
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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.
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