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Author Topic: I GIVE UP!  (Read 10105 times)
N9TLU
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Posts: 35




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« on: November 27, 2010, 12:45:52 PM »

I have a Kenwood TS2000.  I had a fan style dipole for 20 & 40. I was getting some severe "clipping" on 20 meters.  What I mean by "clipping" is that when I transmitted on SSB, my meter would jump from nothing to full like a heart beat and distort my audio so that it was not understandable.  "Arcing" might be a better description. Sometimes it would do it on a dead key on SSB!  My SWR was fine and have a great ground system.  Snap on beads are all over the place in this house!!  I'm not kidding!  Anyway, I put together another fan dipole with the Balun Design model 1115et balun I recently purchased.  Made it for 20-40-80 this time. I pruned and tuned, got everything below 2:1.  Resonant in the middle of the bands.  Guess what?  The "clipping" is gone on 20!  Now I have it on 80!!!!  I have tried EVERYTHING!  Changed grounds, meters, tuners, amps, jumpers. Took each out of line at different times, etc.... No Luck!

I've literally been trying to figure this problem out for a year.  I hate to say this, but I'm actually thinking about giving this hobby up.  I'm a perfectionist and it has actually driven me to that point.

Have I missed something?  I feel like an IDIOT!  Any ideas you might have would be appreciated!

73
N9TLU
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AA4PB
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Posts: 12836




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« Reply #1 on: November 27, 2010, 12:56:26 PM »

Step 1 is to put the radio on a dummy load and see if the problem persists. If its okay then the problem is antenna related, probably RF getting back into the audio.

How far away is the antenna from the radio? What happens if you disconnect the mike and key the radio in SSB? Does moving the mike cable around or running your hand along its length change anything? Have you checked the DC voltage input to the radio when you are having the problem? Maybe RF getting into the power supply.

What is your ground and how is it connected?
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AA4PB
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« Reply #2 on: November 27, 2010, 12:57:43 PM »

One more - how does the radio work on CW mode?
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KI4VEO
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Posts: 166




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« Reply #3 on: November 27, 2010, 01:06:49 PM »

The good ones play...the GREAT ones play hurt.

I agree...it sounds like RF is getting back down the coax and into the radio.

After eliminating the plausible, look to the implausible.  I gave up on fan dipoles because of the interaction problems, but that does not seem to be what you are experiencing.  You might just consider a 270 foot loop and a decent tuner.  The loop is your friend, it works pretty well and isn't too difficult to put up.

If you insist on a dipole, then begin with a single band and add-on.  Try a 40M dipole and use it as a 3/4 wave on 15M.  If it works, and no audio problems, then try adding 80M and see what happens.  It could be something as simple as the proximity of the antenna to the shack or the length of your coax.
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K0BG
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« Reply #4 on: November 27, 2010, 01:22:50 PM »

Sounds like common mode to me. Like Bob suggested, put it on a dummy load, and see what happens.
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AJ8MH
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« Reply #5 on: November 27, 2010, 05:08:48 PM »

Don't give up.

It would help to know your complete configuration.  Since the 2000 is an all mode rig with HF, VHF and UHF, is it possible that RF from your fan dipole is coming down the VHF or UHF antenna feed?  How far is the VHF/UHF antenna from the fan dipole?  Are you using a stock mic or a power mic?  Do you have anything else connected to the radio?  Computer interface?  External speakers?  Do you have any ground loops?  Are you using a single-point ground?  I think the rig has an internal tuner.  If you're using an external tuner, do you have the internal unit shut off?  (I just noticed "amps" in you message.  Are you running a KW?)

I would start by unplugging everything except the HF antenna, power supply and stock hand microphone.  I'd even take the ground-lead off.  Test the radio and if all is OK, start plugging connections back in until the problem starts again.  (You really should start by using a dummy-load for the first test.)

Divide and conquer.

Let us know your progress.  I'm sure we can come up with some additional help.

73,
Joe







 
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N4JTE
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« Reply #6 on: November 27, 2010, 05:27:13 PM »

Feedline problem, replace from antenna to shack.
Bob
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KB9CRY
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« Reply #7 on: November 28, 2010, 03:18:38 PM »

Just use single band antennas.
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N9TLU
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Posts: 35




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« Reply #8 on: November 28, 2010, 05:20:39 PM »

Well, it's RF.  The oil filled dummy load took away all the problems.  Going to try a choke on the INSIDE of the shack for a quick fix.  I think it may be time to reevaluate my antenna choice.  G5RV or trap vertical..  City living sure does complicate matters!!
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WB2WIK
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« Reply #9 on: November 29, 2010, 08:42:53 AM »

Well, it's RF.  The oil filled dummy load took away all the problems.  Going to try a choke on the INSIDE of the shack for a quick fix.  I think it may be time to reevaluate my antenna choice.  G5RV or trap vertical..  City living sure does complicate matters!!

It may be more location related than antenna design related.

Any antenna located too close to the transmitter is likely to cause some problems, especially if you run any serious power (not QRP).  Sometimes taking the same antenna you already have and moving it farther away from the shack solves a lot of problems with no investment other than the time to do it.
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KB1GMX
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Posts: 772




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« Reply #10 on: November 29, 2010, 08:53:08 AM »

There are few if any antennas that multiband (as in 160-10M) without some band that the SWR or common mode problems become pronounced.

First it was 20M with one antenna, I'll bet the coax was a length that interacted with that.
Now it's 80M and I'd bet that the same coax at 20M is now a hot coax(rf on the shield) at 80m.  
Did you try a longer or shorter coax without changing anything else?
Was there a balun used at the antenna for either?  
And what type of balun?

How did you tune the antennas and determine it was  tuned?  I know of one case where the
antenna was way off but the coil of coax feeding it made it look OK, until a longer piece
was used. (with a short coax the swr was >5:1).

How far is the actual antenna from the shack?  Does the problem go away if the power it
cut to 50W, 25W?

Is the radio grounded to an earth ground?  Doesn't always help but sometimes it does.

Is the mic the stock hand held or some other brand add on?
The shield on the audio lead may be longer or less effective and make a better antenna!
Clip a ferrite or three on the mic lead near the set and see if it gets better.

A vertical may do well but only if you have enough ground radials under it. Also you need
a coax line isolator to keep the coax from looking like a radial.  I was driven nuts one
time feeding a HF2V with 45ft of coax (electrical half wave) by RF in the shack due to
it being a magic length. An isolation choke and more radials solved that.

There is much we do not know about you station so there is much to guess about.


Allison
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W8NF
Member

Posts: 53




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« Reply #11 on: November 30, 2010, 05:55:08 PM »

I think you ought to do something to rule out the radio.  Borrow someone else's radio of a different model and see if the problem persists.

All radios have had intermittent issues with correct grounding of the microphone connector at the front panel.  Some never did it right.  The TS-850SAT is nearly a legend for RFI issues because the "ground" for the microphone inside the rig is over a foot away from where the mic connector is mounted.

Dave W8NF
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WB6BYU
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Posts: 13243




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« Reply #12 on: November 30, 2010, 07:45:40 PM »

I've also had problems tuning antennas - especially without an effective balun the coax length affects the
resonant length.  I built one antenna that could be adjusted across the whole 80/75m band, except that
the resonance at 3950 was due to the feedline length rather than the antenna tuning.  No matter what I
did with the antenna, that stayed in the same place.

So it may be that your balun isn't effective enough on 80m, but solved the problem on 20m and 40m.

Also, do you have any external audio connections to your rig, such as a sound card interface?  I've seen
a few issues where RF was getting picked up on external wiring and fed in that way.


And, the truth is, that there is a lot of Elmering that really requires someone there in person to look at your
setup and run some tests.  We can do the best we can, but sometimes there is something obvious that
we just can't see very well across the internet.
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K1BXI
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Posts: 812




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« Reply #13 on: December 02, 2010, 12:38:47 PM »

You didn't say how long your coax was, you might try adding 20-30 ft to see if it breaks up a resonant physical length on 80 meters. Placing a choke balun at a high impedance point on the outside of the coax will not be very successful either. They need to be at a low impedance point.

As others have said, when you have more than one dipole on the same feed line, it becomes a balancing act to get the right length of feed line running away from the antenna in the right direction etc, so that everything works good. Not that it can't be done, it just that sometimes it takes a bit of trial and error to get it right.

Don't give up......you can be successful.   

John
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AA4PB
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Posts: 12836




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« Reply #14 on: December 02, 2010, 12:55:04 PM »

OK, now you know it is RF from the antenna. Next question is how is it getting into the radio. If its common mode current on the coax shield then a choke at the shack entrance might help. Normally a fan dipole is a balanced antenna and shouldn't cause any common mode problems PROVIDED that the coax is not running parallel to and close to the antenna elements. The length of the feed line shouldn't make any difference in common mode. Common mode current is an issue of antenna balance. A good quality current balun at the antenna to feed line connection may help.

The other possibility is that the microphone cable is picking up RF directly from the antenna elements because the antenna is just too close. An RF choke on the microphone cable (near the radio) might help if that is the issue. If you have an other devices connected to the mike input (switch for sound card interface, etc) disconnect it all and connect the microphone directly to the radio.

Try disconnecting other outboard devices from the radio in order to determine how the RF from the antenna is making its way into the radio.

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