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Author Topic: RF in the shack...still  (Read 999 times)
KD8GEH
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Posts: 464




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« on: July 26, 2007, 12:21:51 PM »

Hello all!
 
I've been fighting a rf issue for some time, hopefully I'm doing something obviously wrong. I'm new to HF and maybe missing something.

Equipment is Kenwood TS-950S Digital, Alpha-delta DX-CC at 38 feet flat topped over my barn. Ameritron 811H. No tuner other than the rig.  3 eight foot ground rods (nice buss all equipment tied together).

The rig works fine on the intended bands with close to flat match on 20, 1.5 on 40, higher on the other bands. If I try to run the amp, I get rf popping distortion in the audio .. pretty classic.

I've tried: changing the coax routing, made a 1:1 "ugly" choke per mfg's instructions, Ferrite blocks.  The choke helped a little but not even close to useable. Would a tuner help (better choke?).

Any suggestions? Comments?  Any help is as always greatly appreciated.

Thanks 73  De:  Dave  KD8GEH
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N4CR
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Posts: 1662




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« Reply #1 on: July 26, 2007, 12:26:33 PM »

How far is your antenna suspended from your operating position?
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
KE3WD
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Posts: 5694




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« Reply #2 on: July 26, 2007, 12:56:36 PM »

have you incorporated a 1:1 balun at antenna feedpoint to prevent RF from coming back down the coax (and also radiating, which will distort the antenna's pattern)?  

This could be as simple as the coax coil at feedpoint or could be an actual balun designed for the purpose.  

Proximity of rig to antenna can *sometimes* prove problematic but I'd first suspect the coax if there is no balun up there.  



.
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KD8GEH
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Posts: 464




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« Reply #3 on: July 26, 2007, 01:16:42 PM »

Thanks for both your replies.

The antenna is about 40-45 feet away at the closest point runniny away from the shack (not over). I do have a home brew coax balun at the feed point (8 turns 10 in diameter). Maybe a store bought might work better.

Interesting point, the coax isn't the best, rg-58/U.  I dont have the problem with the 10 meter antron vertical with RG8. Would you guys suggest:

*Moving the antenna? Problematic, It would take alot of coax to use other trees and would just look horrible.

*Try another type antenna? Like? I like all the bands...

*Try better coax? Would RG8X, RG8 or LMR400 be best?

Thank you both,  73 Dave KD8GEH
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W5GA
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Posts: 430




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« Reply #4 on: July 26, 2007, 01:37:37 PM »

put another "ugly" choke right outside your house where the feedline comes inside.

Can you describe what the instructions say about this choke?

Do you have any ferrites on the radio power cord?
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WA3SKN
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« Reply #5 on: July 26, 2007, 01:43:34 PM »

How long/ far away is your ground run?

-Mike.
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K5LXP
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« Reply #6 on: July 26, 2007, 02:00:17 PM »

> Alpha-delta DX-CC
...
> No tuner other than the rig.

If you don't have a tuner between the amp and the non-50 ohm antenna, how are you providing a match?  Sounds like you are driving an unknown/unmatched Z with the amp.


Mark K5LXP
Albuquerque, NM
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AB8SG
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Posts: 136




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« Reply #7 on: July 26, 2007, 02:00:18 PM »

If this is a fan dipole, check to see if one side or both has tangled. Same symptoms happened to mine, caused by twisting in the wind. The unbalanced elements then caused the rf problems when using the amp. Also check the soldering in the amp metering circuit. Mine had a lossy connection at the circuit board between the bridge resistors. Result was causing erroneous meter readings which leads to improper loading.
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K4SAV
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Posts: 1837




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« Reply #8 on: July 26, 2007, 03:16:13 PM »

Could be a bad coax connector on a jumper cable.  Try changing out the jumper cables.  Could be a bad shield on the mic cable. Try a different mic, or try wrapping a few turns of the mic cable on a ferrite bead.  If your mic goes thru any equipment before going into the radio mic input, remove the other equipment.  Make sure you have a ground strap between all pieces of equipment on the shack desk.

Jerry, K4SAV
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W6OP
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Posts: 338




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« Reply #9 on: July 26, 2007, 03:30:28 PM »

A ten inch coil isn't going to help much above 40 meters. See my article on current chokes at http://www.eham.net/articles/15383
Also, look at K1TTT article at http://www.k1ttt.net/technote/airbalun.html and also another at www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf

Pete W6OP
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K9ZMD
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Posts: 170




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« Reply #10 on: July 26, 2007, 04:13:18 PM »

Dump the RG-58 in favor of RG213 if you plan to continue using that amplifier; anything bigger than RG-58 would be better (somehow I suspect you already knew that, but have been pushing your luck). A good coaxial choke should greatly reduce the RF common mode currents on your transmission line.  One at the feed point, and one where the feed line enters the house would be helpful to reduce your in-shack RFI.  Please consider the following:

1.  "Ugly" coax choke implies scramble-wound rather than flat, side-by-side windings of coax in the choke.  Scramble wound is not very predictable, and yields chokes that are less effective than when windings are evenly laid side-by-side.

2.  For a well-made RG-58 choke (not scramble wound) of the dimensions you provided, your choke impedance would be in the neighborhood of 1.9KOhms at 7 MHz, 8.9K Ohms at 10 MHz, 4.5K Ohms at 14 MHz, 2K Ohms at 18.1 MHz, 1.5 K Ohms at 21 MHz, 1.1K Ohms at 24.9 MHz, and 0.9 K Ohms at 28 MHz. The coaxial choke would have a working resonant frequency at about 11.3 MHz, where the insertion loss will be very high.  Fortunately, that peak is steep and subsides quickly, allowing an acceptable insertion loss at the ham bands.

3.  To get better choke impedance on 10 meters (where 0.9K Ohms is marginally effective), try just 7 turns of RG-58.  That moves the working self-resonant frequency to about 12.8 MHz (so the insertion loss peak is still not in a ham band) and gives you a 10 meter choke impedance of about 1.0K Ohm, and 40 meters will still have a usable choke impedance of about 1.4K Ohms.

With all that said, I've arrived at those choke figures using a simple calculator designed by Reg Edwards, G4FGQ in 2004.  Find it at http://www.smeter.net/feeding/selfres3.php  

You can do the coaxial choke design work yourself with the calclulator - just plug in the specs and dimensions you want to work with.  I have been experiencing good luck using the calculator to design coax chokes, but those with more experience may see considerations that rule against it.  In that case, let me know so I don't lead anyone astray.
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AA4PB
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« Reply #11 on: July 26, 2007, 05:11:24 PM »

The antenna is about 40-45 feet away at the closest point runniny away from the shack (not over)
----------------------------------------------------
Does this mean that you have a long run of coax running parallel to the antenna? If so, this may very well be the problem - coupling between the antenna and the outside of the coax shield.
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W8JI
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« Reply #12 on: July 27, 2007, 04:03:09 AM »

Dave,

A ground system or ground rods is not needed at all for RF proofing. It is a big giant Ham radio myth that people keep repeating. It is, unfortunately, wrong. The shack ground is really just for electrical safety, unless you are feeding a long wire or Zepp or Windom from the shack.

The RF in your shack that causes RFI comes from a differential between things entering or in your shack, NOT from the lack of a RF ground. It often isn't even RF.

There are two ways RF causes a problem:

1.) The antenna is too close to the shack and induces RF directly into wiring

2.)The antenna design or installation is poor.

The single most important thing is all pieces of gear bond to a common point that is bonded to any wiring entering the operating area. You should get an outlet strip that has a ground post (many metal housing strips do this) and have every wire powering things at your operating position common to the point, and all feedlines entering common to that point.

Second, make sure the feedline is good and the antenna design is good. Adding a hunk of coiled up coax, despite what you read, might actually make things WORSE. This is because the coiled up coax adds reactance to the shield, and if the sign of that reactance is OPPOSITE the common mode impedance reactance it can increase common mode currents!!! This can be true no matter how many turns you add. Because of this you might need ferrite beads that are primarily resistive, or you might need to experiment with choke location or number of turns.

Also be SURE you don't have a computer audio interface, or if you do have one be sure it isolates the ground. You NEVER want any audio line to be grounded at both ends of a shield. This means a direct connection from the mic input of your radio to ANY other chassis or cabinet would be a disaster.

The least important thing in any of this is the lead down to the ground rods and the ground rods. The ONLY time a Hamshack needs a RF ground is when something is wrong with the wiring or the antenna design.

The ground is needed for safety.

73 Tom






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WA3SKN
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Posts: 5457




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« Reply #13 on: July 27, 2007, 04:45:20 AM »

OK, RF is getting into the Mic circuit.
First, replace the antenna with a dummy load.  Do you still have the problem?
Of course, check the ground and coax connections.  RG58 is has a maximum power rating of 400 watts, properly terminated.  You might want to upgrade to RG213,etc.
If it works OK with a dummy load at the far end of the coax, then you can examine your antenna arrangement for possible improvements!

-Mike.
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AD5VM
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Posts: 26




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« Reply #14 on: July 27, 2007, 08:11:45 AM »

RG-59 bad.
RG-213 good.

MFJ sells RF isolators, big PVC thing with like 50 chokes on a 1 foot run of RG-213. Put this after the output of your amp.
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