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Author Topic: VHF Beam troubles  (Read 298 times)
KA3NXN
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« on: March 22, 2006, 07:15:19 PM »

I assembled a Cushcraft 10 element beam and had it mounted 4' off the ground while I tuned it. There were no obstructions around and the SWR was 1.3:1 at 145.52MHz. This was great. I put it up on the mast on the roof and the resonant frequency dropped 5MHz. Now the SWR is flat down at 141.5MHz and at 146.52 it shot up to almost 3:1. The other pecular problem, is that even where it is resonant,  I talked to some stations that with a 1/2 wave vertical I used to give an S7 signal, with this thing I am only giving them an S2. So the problem is 2 fold. 1) the SWR shot up where it should not have. 2) Even with the antenna pointed at certain stations, the signal it is putting out is very very reduced compared to a simple 1/2 wave vertical. The only other thing close to this antenna is that I have a 11 element 440 beam about 3' above this one.

I will be going back up on the roof this weekend. What should I be looking for other than the obvious, retuning, loose connector...
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WA9SVD
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« Reply #1 on: March 22, 2006, 08:07:44 PM »

Is the new beam antenna oriented vertically or horizontally?

    Cross polarization can cause 20-30 dB (or even higher!) loss.
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N3BIF
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« Reply #2 on: March 22, 2006, 10:30:10 PM »

I hesitate to bring this up but often we forget that we need to point the beam at the repeater not the other station if working other than direct.
   If working direct then first use the beam and peak the receive on the station you are working and then do your AB if you can. Don't pay any mind to where you think it should be pointing at this stage of the game.     See if there is improvement or not. I know from experience that up to about 30 miles my beam is worse then my Ringo vertical but after that is when the beam shows its value.
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K5DVW
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« Reply #3 on: March 23, 2006, 04:50:45 AM »

Thoughts... if your SWR is 3:1 on VHF at the end of an appreciable length of RG8 or smaller cable, then you may have a very bad mismatch at the end of the feedline. This situation will burn up most of your TX power in the feedline rather than radiate it.

My vote is that something has become disconnected, or maybe you disturbed/didn't secure the matching structure when you mounted the antenna. I've forgotten to bolt down the gamma match hardware only to have it slide around later.

Also, if it's mounted vertically, hopefully the antenna mast is BEHIND the reflector element. I'm sure that's obvious, but it never hurts to mention it.
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KA3NXN
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« Reply #4 on: March 23, 2006, 05:27:24 AM »

The antenna is vertically polarized. Unfortunatly because of the lenght of the boom, it attaches to the mast right in the middle of the boom. But I tuned it this way. I had it on a metal boom on the ground when I tuned it.
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K5DVW
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« Reply #5 on: March 23, 2006, 06:53:37 AM »

Having a vertical metal mast in the middle of a vertical yagi is asking for trouble. No matter what the SWR is, it leads to severe pattern distortion and reduction of forward gain. Infact, as you rotate the antenna around, you might find that maximum signal is no longer off the end of the antenna as it should be.

SWR has nothing to do with antenna pattern or  efficiency, so really it doesnt matter if you've tuned it to look good with the mast there.

If possible, can you transition to a wood or plastic mast extention going thru the center of the antenna?
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WB2WIK
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« Reply #6 on: March 23, 2006, 07:56:41 AM »

You can't put another conductive-boom beam only 3' above or below a vertically polarized 2m yagi.

You *can* space 2m and 70cm beams only 3' apart, no problem, when they're both horizontally polarized.  But when using them vertically polarized, this places the 70cm boom only 16" away from the element "tips" of the 2m vertically polarized beam -- *waaaay* too close, because it is the element tips that have the big electric field and they detune very easily when placed in proximity to anything conductive.

I'll bet when you were tuning this up on the ground the 70cm beam wasn't there.

WB2WIK/6
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K0RFD
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« Reply #7 on: March 23, 2006, 10:27:12 AM »

It's generally best to tune an antenna at its operating height. That way you're taking into account the actual capacitive coupling between the antenna and the ground when you tune. Tuning can change quite a bit when you move the antenna from near-ground to its actual operating height. If you MUST tune the antenna on the ground, try pointing it straight up.  This may or may not work, and certainly won't work 100 percent, but people have been doing it for years.

DVW is right that you're probably burning most of your power in the feedline.  And although he's also right that SWR itself doesn't affect the antenna pattern, element length and spacing DO affect the antenna pattern.  If you tuned your antenna on the ground and the element lengths are wrong, then the same problem can be causing both symptoms.

Try tuning it at operating height.  If that's not practical, tune it as close to operating height as possible.
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WB6BYU
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« Reply #8 on: March 23, 2006, 12:58:52 PM »

If you tuned the beam originally close to the ground (and
4' is just over a half wavelength - considerably less than
the boom length) then the ground will affect the impedance.
The best way to do this is to aim the beam straight up
which minimizes ground interaction.


So at least three things to do to improve your antenna:

1) retune the antenna pointing up rather than parallel to
the ground.

2) increase the spacing from the 440 beam

3) get that metal mast out of the pattern.  Either use a
non-conductive mast or mount the two beams on the ends of
a horizontal cross-boom so they are a few feet out from
the mast.

4) And while you are at it, make sure that the coax
cable comes off the back of the boom, out of the field
of the elements, rather than running vertically among
the elements.
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KE5GBY
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Posts: 59




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« Reply #9 on: March 23, 2006, 04:20:59 PM »

 "Having a vertical metal mast in the middle of a vertical yagi is asking for trouble."



I have a question. I have a Cushcraft 148-20T dual polarized 20 element VHF beam mounted on a 5' metal mast that goes into my rotator, and all of this sits on top of a 30' telescoping pole.
I use this antenna for both SSB and FM, which is why I bought it.
I have not had any problems with it, and set the gamma match's per the intruction manual. I do not have a SWR meter.
Should I NOT use the 5' metal mast for this antenna? Even though I am not having any problems, might I be missing out on some performance?
The manual said nothing about NOT using a metal mast.


-Bill
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K5DVW
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« Reply #10 on: March 24, 2006, 05:14:54 AM »

>Should I NOT use the 5' metal mast for this antenna?

I just can't help but think it would improve things on the vertical plane if you didn't have a metal mast running between the vertical elements. The Cushcraft manual shows a photo of the antenna with a metal mast, so who knows! Then again, I've seen HF vertical antenna instruction manuals from the same people that say you don't need to use radials.

Whether it's worth changing, I couldnt tell you. If you can find a distant repeater in use, just sweep your antenna around in azmuth and note any signal strength changes. That might answer your question. If you have pattern distortion, you'll have sidelobes off the sides of the antenna and not a single maximum straight off the end, as it should be.

Good luck with it!
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KE5GBY
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Posts: 59




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« Reply #11 on: March 24, 2006, 08:09:27 AM »

"If you can find a distant repeater in use, just sweep your antenna around in azmuth and note any signal strength changes."


Well I do know that when there is a net on a repeater about 25-30 miles away and I move the beam the signal will improve when I have it pointed in the right direction. If I move it a little the signal will drop off.
Also, the mast is not centered on the beam. Per the instruction manual it mounted closer to one end of the beam.
Sometimes I try and hear a guy on reverse that is a regular on our local repeater who is about 150 miles away or so. When I point the beam in his direction the signal will either be there or not when I am "sweeping".
So I hope this means that the beam is operating like it is supposed to on FM.
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