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Author Topic: RFI in the shack question  (Read 523 times)
KA3NXN
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« on: April 13, 2006, 08:57:46 AM »

I have a 2nd floor shack that I have run a pretty extensive RF ground and I am using a TEN TEC counterpoise tuner to maximize the effect of the ground at different freuencies. The initial problem was that I am suffering from RFI especially when I kick the amp in and reach the 1KW mark. I have an Icom 746 Pro with an Alpha 76A amp, and a Palstar 1500CV tuner. On 20 & 17 Meters I get RF bites with the amp on if my lip accidently touches the mic while I am transmitting. (OUCH!) On these bands it also causes my 220 rig to go crazy when I am transmitting. On 40 meters I have to really turn the power down because my audio sounds very raspy on the other end and I am being told that I have stray RF in my audio. I also hear my very distorted audio coming from my speaker in the shack.  The grounding did help, but did not completely eliminate the problem. I would appreciete any other suggestions to cure the issue. I was thinking of coiling the coax at the transmitter end and at the antenna end. I was told that 10 turns about 6" in diameter should choke out the coax from radiating. Oh yea one minor detail... The antenna that I am using is the Radio Works Carolina Windom 40.
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WD8PTB
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« Reply #1 on: April 13, 2006, 09:10:45 AM »

coiling the coax at both ends should help. You can also buy line isolaters, they do about the same thing as coiling the coax but probably do it better and sometimes you do not have enough coax to coil up, especialy for the lower frequencies. Make sure you tune your artificial gnd when you change bands. Sometimes a resonant radial hooked up to your rf ground will help. Line isolaters and coils of coax may also help between your radio and amplifier. Good Luck; Don WD8PTB
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K5DVW
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« Reply #2 on: April 13, 2006, 09:25:49 AM »

I think the Windom is more than a minor detail and is what's causing your problem.

The Windom is an off center fed dipole, and is prone to create feedline radiation. That's the nature of the beast. Putting a choke, balun, or coiling the feedline out near the antenna might actually help and is worth a shot.

Just curious how you have an extensive RF ground on the 2nd floor? You might do better to have the RF ground OUTSIDE where the coax enters the house. A basic RF ground consists of several long buried radials. A tuned counterpoise on the 2nd floor works well for making your SWR match look good, but it also radiates, sometimes as much as the antenna does.
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WIRELESS
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« Reply #3 on: April 13, 2006, 09:29:33 AM »

It sounds like you are using a long wire antenna?Huh? If you are then convert it to a dipole any way possible.  Long wires are notorious for rf in the shack because it is in your shack since the beginning of the antenna is in your shack.  

IF you are using a dipole, something is unbalanced or amiss.  Describe your antenna and maybe some clues will emerge.
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KA3NXN
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« Reply #4 on: April 13, 2006, 10:07:26 AM »

I am using a Radioworks Carolins Windom 40, as stated in my original post. see: http://radioworks.com for details on this antenna. Basically it's an OCF dipole with a vertical radiator.

I do have my ground running outside to 6 ground rods  burried in the ground and I am running 1/2" braid in a star configuradion. This runs to the counterpoise tuner and then to the radio gear.
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K5DVW
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« Reply #5 on: April 13, 2006, 10:47:38 AM »

My first impression is that being 8 to 10 feet from the ground makes your ground system nearly ineffectual at RF frequencies and you have a ton of feedline radiation. My guess would be that even with the ground tuner making things appear normal, the wire between your ground rods (not such a hot RF ground by the way) and your station is radiating and so is your feedline! I think this was what was meant by "longwire antenna" since it's the same problem you would see feeding classic longwires.


For your Windom, do you bring the vertical wire into the shack or does it transition to coax with a  ferrite sleeve balun or something?

If it's coax fed, what might improve your situation is to put an RF ground in the form of ground radials connected to the coax shield on the radio side of the ferrite sleeve. That ferrite sleeve is really the antenna feedpoint since it relies on the vertical coax portion to actually radiate. The RF ground really wants to be at the feedpoint.

If you dont have a ferrite sleeve (also called a decoupler) then you need one for this type of antenna.

Think your antenna as a form of top loaded vertical. Now, the same type of radial rules apply.

That's my stab at it without actually seeing what you have. Hope it helps.



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K0RFD
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« Reply #6 on: April 13, 2006, 11:38:58 AM »

I think the Windom is part of the problem.  Good antennas but sometimes tough to tame.  The Radioworks Windom does use a line isolator, but the manufacturer recommends that the 40 be installed at heights greater than 30 feet.  They say it will work at 25 feet, how high is yours? Remember the Windom 40's vertical radiator needs to come away from the OCF dipole at right angles.  If it doesn't that could definitely be another part of your problem.  

Also, how close is the antenna to your shack?  

Radioworks is pretty specific about using 86 feet of RG-8X feedline.  Probably to find a "sweet spot" at the tuner end.  Are you using 86 feet of feedline?
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WB6BYU
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« Reply #7 on: April 13, 2006, 12:30:27 PM »

Bring the coax from the antenna directly to your ground
system and ground the coax shield at that point.  (A lightening
arrestor will do the job, or a SO-239 through a metal plate
clamped to the ground stake.)  Add a decoupling choke
(ferrite or wound coax) in the lead from there to your
station.  This should work as long as your ground rods are
making good contact with the soil and not just buried in a
few inches of dry sand.

Or connect a quarter wave counterpoise wire to the lug on
the back of your tuner for each band you want to operate
and string it around the walls.

Also make sure that your mic cable - and any other connections
to the back of the transceiver - is well shielded and/or
bypassed.  Some rigs are known for having unbypassed
audio inputs on the rear panel, and it doesn't take much
RF (even radiated from the antenna) to get into the rig
through these connectors, especially with unshielded wire.
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KA3NXN
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« Reply #8 on: April 13, 2006, 01:25:15 PM »

So far what has worked the best is WB6BYU's suggestion to use a lightening arrestor to ground the braid to my ground point. I am now able to bring the power up to about 1100 watts without getting the raspy audio on my signal.
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KA3NXN
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« Reply #9 on: April 13, 2006, 01:25:44 PM »

So far what has worked the best is WB6BYU's suggestion to use a lightening arrestor to ground the braid to my ground point. I am now able to bring the power up to about 1100 watts without getting the raspy audio on my signal. The 220 rig is still going nuts though
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WB6BYU
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« Reply #10 on: April 13, 2006, 04:05:19 PM »

That was quick!


The counterpoise tuners might work with short wires that
are not grounded at the far end, but there are many situations
where they will certainly NOT give a good ground.  The
main problem is that they just cancel the reactance of a
wire but don't transform the impedance.  If a wire is anywhere
close to half a wavelength long (or a quarter wavelength
if it is grounded at the far end... like the wire going
to a field of ground rods) then the impedance will be very
high even when the reactance is tuned out.

A quarter wavelength wire, on the other hand, has a reasonably
low Q so tuning in this application isn't as critical.
Shorter wires can be tuned to have a low impedance (limited
by the wire length and the Q of the components) but may
require retuning more often when you QSY.

So besides checking for a good ground connection on the
shield of your microphone cable, I'd suggest removing the
"ground wire tuner" and connecting your external ground
system directly to your rig.  See how that works.  If you
still have a problem on some bands, install a quarter
wave radial wire in the shack connected directly to the
rig.  If you can't manage a quarter wave down the back
of the desk and around the edge of the floor, then use
a shorter wire and the ground wire tuner.

Oh, and check the ground connections to the coax connectors
in your feedline - a bad ground to the shell of a PL-259
is a common source of problems because it causes LOTS
of RF to flow back on the coax shield.
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K0ZN
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« Reply #11 on: April 13, 2006, 08:31:46 PM »


Hi,

I am a little late to the party, but count me in with the "It likely is the Carolina Windom" more than anything else.  Nothing personal, but that antenna is "rated" to do WAY more than it "does" do. There seems to be as much myth as there is theory in that antenna. No doubt it does work well on some bands but on other it "defies" antenna theory...and you then have problems.

Bottomline: try to come up with a balanced antenna system...a Center Fed Zepp/Doublet or some multi-wire coax fed dipole, a ladderline fed Loop, etc. I think your problems will magically disappear with a balanced antenna system.

73,  K0ZN
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