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Author Topic: High SWR on Hy-Gain TH3MK3  (Read 4497 times)
VA3GUY
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Posts: 172




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« on: October 08, 2012, 06:58:46 AM »

I put up a Hy-Gain TH3MK3 on my roof the other day (approx 35') and assembled it according to the plans for the CW portion of the bands.  I checked and double checked the dimensions, when assembling the antenna, to be sure they were spot-on.  I am using a COMTEK, W2FMI Series BAL-11130T, Dual Wire/Single Core, 1:1, 1.8-54 MHz, Weatherproof balun and approx 50' LMR400.

On 10 meters between 28.010 MHz and 28.200 MHz, the swr is 1.5:1 and 1.7:1.  On 15 meters between 21.010 MHz and 21.075 MHz, the swr is 1.4:1 and 1.1:1.  On 20 meters between 14.010 MHz and 14.075 MHz, the swr is 1.5:1 and 2.0:1.

I would like to lower the swr from the 2.0 range if possible.  I assume that if I make the length of the elements longer, that should bring the swr down?  Or maybe change the location of the beta-match?  Any other suggestions?

Thanks and 73...
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KA4POL
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Posts: 1914




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« Reply #1 on: October 08, 2012, 07:19:24 AM »

The SWR is in an acceptable range. I would not change anything. I hope you don't think it has to be 1:1 everywhere.
Attenuation with your cable is .35 dB for 50' at 30MHz. You should be fine.
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WX7G
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Posts: 5920




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« Reply #2 on: October 08, 2012, 07:55:24 AM »

Check the VSWR across each band to find where the minimum is. If it is way up in the phone band then you might think about tuning the antenna. But if the minimum VSWR is the CW band there is no problem. As KA4POL says the VSWR is acceptable.
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VA3GUY
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Posts: 172




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« Reply #3 on: October 08, 2012, 07:58:14 AM »

The reason I want to lower the swr is that when it hits 2.0, my rig drops the power output down from 100W to around 70W.  The rig is an IC-7200 and I know it drops power output down to protect itself.  I figure if I can get the swr down, I will have better output results.
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KA4POL
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« Reply #4 on: October 08, 2012, 11:03:13 AM »

The manual for the 7200 says it starts lowering the output at about 2:1. But that's the manual. So you need to find out where you are on the SWR/frequency graph. For that you have to follow the procedure which WX7G described.
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WX7G
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« Reply #5 on: October 08, 2012, 11:31:50 AM »

If the minimum VSWR in the band is as you describe it's time for an antenna tuner.
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K8AXW
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Posts: 3669




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« Reply #6 on: October 08, 2012, 09:22:34 PM »

GUY:  Prior to installing my TH6DXX on the tower, I set all dimensions according to the assembly manual..... to within a 1/16 inch.  When I got the beam in place the SWR was nowhere near where it should have been.  I almost stroked out!

Long story short, go to the tuning procedure in the assembly manual and adjust the antenna elements following their recommendations.  I forget now rather you tune for the lower bands first or last...... but there is a proper way to do it.  (Similar to a multiband dipole antenna)

While the SWR on 15 and 10 are fine, I couldn't handle the SWR you show on 20m either.  Especially if the transmitter power is folding back.

If you can't make heads or tails of the printed instructions, don't hesitate to call the manufacturer.  They'll be happy to assist you.  Fine tuning a beam is a royal PITA but if you're anything like me, it has to be done!
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K4RVN
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« Reply #7 on: October 08, 2012, 10:31:05 PM »

Check to see where the SWR is the lowest by tuning up or down the band. If it is lower up toward the phone portion, then
increase the length of the elements to lower the frequency if you want max output power for CW. I have a 7200 and I sure do miss the antenna tuner I had built into my Kenwood 570DG. You might consider a small tuner so you can operate in the phone and CW Frequencies at full 100 watts out as someone else also suggested.
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W8JI
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« Reply #8 on: October 09, 2012, 05:01:35 AM »

How long are the leads on the balun to the antenna element?

The wires from the balun to the antenna elements are part of the element length. Not only that, the wires inside the balun box, the screws through the wall of the balun, and everything else that is not a 50 ohm transmission line affect the 50 ohm SWR.

My guess is, from the numbers you posted, the antenna is resonant below the 20 meter band. This could be a combination of many things, but I would especially look at the lead lengths between the balun and the element. This problem is increased by using thinner wires between a balun and thick antenna element made from tubing. You usually cannot get the wires close together and the tubing and mounting get in the way.

How long are the leads?

73 Tom
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K8AXW
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Posts: 3669




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« Reply #9 on: October 09, 2012, 09:19:35 AM »

GUY:  Tom makes a valid point.  When Hy-Gain builds their antennas and provides instructions for adjusting the element lengths and then you change the balun that they recommend, then all bets are off.  The same applies to where and how you mount the balun.

Your balun will no doubt work but you might have to make adjustments.
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VA3GUY
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Posts: 172




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« Reply #10 on: October 09, 2012, 09:17:07 PM »

TOM:  The balun leads are approx 6" long from the balun to the driven element and another 6" from the driven element to the beta match.  I have a Hy-Gain BN-86 here but I was told that it is a voltage balun and is not recommended, hence the purchase and use of the COMTEK.  By the way Tom, I was looking to buy one of your baluns but I needed one that goes to 54 MHz because I am going to be adding a 6 meter parasitic array to the boom so I will need the higher frequency capability.

FRANKLIN:  I have a transmatch here (built by VE3GK - if any of you knew him) and have tried it on a few bands other than 10-15 & 20.  I did not get good results so I assumed that a tuner is not supposed to be used with a trap antenna.  True or not true?

ALLEN:  Where, exactly, do you find the tuning procedure in the manual?  I have the assembly instructions, beta match assembly and final assembly but nowhere do I see anything to do with adjusting the elements in any specific order.

ALL:  When I assembled the antenna, I did so with very accurately for the CW portion of the bands.  So I guess I could increase the length of the elements a bit more but I don't believe there is too much more play in them.

Thank you for all replies/suggestions.  Keep 'em coming!

73,

- Guy
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K4RVN
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Posts: 758




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« Reply #11 on: October 09, 2012, 09:27:47 PM »

You should be able to use your tuner with the hygain. Also I checked the manual for your old antenna and the ends can be adjusted without moving the trap lengths. I would shorten the ends
if not too much trouble to bring the frequency up if you find it is too low. If you have or can borrow an antenna analyser like a MFJ 259, etc. it would maybe give you a clue. I still believe that your tuner will let you put out 100 watts on 20 meters and it won't hurt to try that. The leads you mentioned should be as short as possible as the manual calls for about 7.5 inches I believe for the coax leads.
Going from memory on that so not sure. Tom may have some good info on that when he reads how long your leads are. He sure knows more than I do.

Frank
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WB6BYU
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Posts: 13027




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« Reply #12 on: October 09, 2012, 09:29:51 PM »

Look carefully at the drawings in the manual and estimate the lengths of the
connecting wires, and try to use the same length or shorter.

Any extra length in the beta match will change that setting as well.

That's why it was suggested that you measure the SWR across the band:  if it
dips to a minimum value some place out of band, then you need to adjust the
element lengths.  If it dips in the band but not low enough, then you may have
to readjust the beta match to account for the added wire length.  It is a
compromise among all three bands, so don't just set it for 20m and assume it
will work properly on 10m.  (There is some interaction between the settings,
but this gives you a sense of where the problem may be.)
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G3RZP
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Posts: 4391




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« Reply #13 on: October 10, 2012, 02:21:54 AM »

It must be me, but I've never put a Hy-Gain antenna together to the dimensions and had it right. Always a little bit off to the HF of where I wanted it. When I got the 205BA right, in spite of being a bit short on boom length, it went really well. My only reason for changing to a 4 ele SteppIR was to get 17 and 12 (plus the £/$ ratio happened to be good at that time!)

But the basic TH3 is a very old design going back to the days of tube PAs that were far less susceptible to SWR, and, as WB6BYU says, it's going to be a compromise between all the bands.
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W8JI
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« Reply #14 on: October 10, 2012, 08:10:58 AM »

TOM:   I have a Hy-Gain BN-86 here but I was told that it is a voltage balun and is not recommended, hence the purchase and use of the COMTEK.  By the way Tom, I was looking to buy one of your baluns but I needed one that goes to 54 MHz because I am going to be adding a 6 meter parasitic array to the boom so I will need the higher frequency capability.

Guy,

I'm not sure who told you that, but the BN86 is NOT a voltage balun. The BN86 is a current balun.

Quote
The balun leads are approx 6" long from the balun to the driven element and another 6" from the driven element to the beta match.

I'm not entirely sure what you mean by lead lengths, but six inches is significant (especially with thin wires). That's like adding somewhere around one foot, maybe more, to the system.  Feedpoints on Yagi's are pretty fussy, so it is best to do things exactly as the antenna manufacturer recommends.

You can adjust driven element lengths to tune the antenna to the center of the operating range, and this will help compensate for long thin leads. Beta match lengths and attachment methods are still very sensitive.  The general rule about something not being critical is one electrical degree. That is 1.5 feet on 160 meters, and 2 inches on 20 meters.

Let me give an example....

Inside amplifiers we have T/R relays. The lead length through a typical small DPDT relay is about three inches total. That lead length can cause a flat load to show 1.3:1 SWR or higher on ten meters, but has no noticable effect below 20 meters.

This is why we either should be careful to do what the manufacturer says, or we should be prepared to readjusta little.

Do an SWR sweep on the antenna and see whether the best SWR is too low or too high, and then you can adjust the driven element length in logical steps.

73 Tom

 
 
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