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Author Topic: Tuning a Hamstick dipole with a tuner  (Read 617 times)
AG4DG
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« on: November 01, 2006, 11:18:45 AM »

I have a 75m Hamstick dipole and an MFJ-945E tuner/SWRmeter.

As expected, the 75m Hamstick dipole has a very narrow bandwidth.  In fact, being just 40 kHz away from resonance means an SWR of 4 or more instead of under 1.5.

I know that you're not supposed to use a Hamstick on a band different from the one it's designed for.  But my MFJ-945E tuner can't reliably bring down the transceiver SWR even when I'm just a few tens of kHz away from the resonant frequency.  Do 75m Hamstick dipole users go out and readjust their antennas every time they want to QSY just 10 kHz?  

Should I just readjust my 75m Hamstick dipole for 3980 kHz instead of the current 3940 kHz given that it's Iowa's SSB emergency net frequency and simply live with the high SWR in all other parts of the 75m band?
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AG4DG
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« Reply #1 on: November 01, 2006, 11:20:39 AM »

One more thing to add: The tuner can't reliably bring down the SWR as seen by the transceiver because of the hair trigger sensitivity.  
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KB9CRY
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« Reply #2 on: November 01, 2006, 11:40:18 AM »

The DX Window is 3790 - 3800, that's where you should be.

Yes, your shoes will be getting a workout.
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AG4DG
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« Reply #3 on: November 01, 2006, 11:45:30 AM »

< The DX Window is 3790 - 3800, that's where you should be. >
Except that a low Hamstick dipole radiates mainly straight up.  That's good for NVIS (which is what I want for this particular antenna) but not for DX.
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WB2WIK
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« Reply #4 on: November 01, 2006, 11:47:30 AM »

3980?  Holy cow, I never even listen up there!

I'd never tune my antenna for 3980...that's almost out of the band.  

My 75/80m dipole is tuned for 3800 and is useful across the entire band, but below 2:1 from about 3720 to 3880.

Not surprised your tuner can't handle the highly reactive, very low resistance load of the Hamstick dipole.  It's a tough job for any tuner.  Can you try a loop?
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KB4QAA
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« Reply #5 on: November 01, 2006, 02:17:08 PM »

I think that you are discovering the realities of using whips and such.  They compromise almost every bit of performance to fit a small size.

Since you mention the tuner being incredibly touchy, one possibility is that you may be getting some RF back  into the tuner from the coax radiating.  

Do you have a 1:1 balun or isolator in the feed line between the tuner and the antenna?   If you don't want to buy one, you can take about 6 turns of the coax around a coffee can, remove the can and tape the loop together.  This makes a choke to prevent the RF from traveling down the coax shield.

good luck.

bill
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NS6Y_
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« Reply #6 on: November 01, 2006, 02:28:03 PM »

A balun will help a lot.

Also, you're learning a fact about antennas - with most of them, a good antenna will mean a narrow bandwidth. Beware of whips like the Miracle Antenna that are broadband.

There are some designs that are more broadband, like the LPDA or discone, but in the real world of whips and dipoles, narrow bandwidth is a good sign.
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WB2WIK
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« Reply #7 on: November 01, 2006, 02:40:58 PM »

>RE: Tuning a Hamstick dipole with a tuner    Reply
by KB4QAA on November 1, 2006    Mail this to a friend!
I think that you are discovering the realities of using whips and such. They compromise almost every bit of performance to fit a small size.

Since you mention the tuner being incredibly touchy, one possibility is that you may be getting some RF back into the tuner from the coax radiating.

Do you have a 1:1 balun or isolator in the feed line between the tuner and the antenna? If you don't want to buy one, you can take about 6 turns of the coax around a coffee can, remove the can and tape the loop together. This makes a choke to prevent the RF from traveling down the coax shield.<

::Not on 75 meters, it won't.  A coaxial RF choke balun as described works great at higher frequencies, 20-10 meters.  On 40m, you need more inductance than that and on 75m you need a whole lot more inductance than that.  (Inductance being the inductance created by making a coil out of the shield of the coaxial cable, of course -- the coax remains 50 Ohms no matter what you do).  The whole idea of making a "choke balun" is to raise the impedance of the surface of the cable's outer conductor to something several times higher than the antenna impedance.  To do that on 75m, it normally takes 20 turns of coax on a 6" form as a minimum.  Because that's so clunky (heavy, ugly), most people don't bother trying to use choke baluns on such a low frequency, and stick with other approaches, like copper wire wrapped around a lot of ferrite.

Frankly, I doubt that will help in this situation.

WB2WIK/6
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AG4DG
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« Reply #8 on: November 01, 2006, 02:41:26 PM »

I'm using two Hamstick dipole brackets in parallel (one for 40m and one for 75m) and one Radio Works unun in series to maximize common mode impedance.  So I don't think common mode current is an issue.
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WB2WIK
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« Reply #9 on: November 01, 2006, 03:20:54 PM »

Did you try changing the length of the coaxial feedline?

Of course on 75m, you have to change it by forty feet to accomplish a 90 degree shift, so changing the line length by a couple of feet one way or another won't do anything...
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W5DXP
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« Reply #10 on: November 01, 2006, 03:51:05 PM »

> AG4DG wrote: I have a 75m Hamstick dipole ...<

Bad idea. Hamstick dipoles work pretty well on 20m-10m. They are less than 1% efficient on 75m. 100 watts in - less than one watt out. Is that what you really want?
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KE6VG
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« Reply #11 on: November 01, 2006, 06:38:57 PM »

The problem is your dipole should be tuned for 3515. Everyone knows that's where real men hang out.

To get serious about things though. The hamstick on 75m is only going to work on one frequency unless you retune it for another. For portable/emergency work this is fine. I don't know what kind of room you have for antennas or how or where you are going to use this.

At my last house I had a full sized dipole up for 80m up on the roof. I only had enough room up their for maybe a 40 meter dipole. First I tried some home made loading coils on the end of the 40 meter dipole. It worked and gave me about 80Khz of bandwidth under 2:1. Then, I put up a 15' swimming pool pole as a mast. You can buy these almost anywhere for under $20. I kept the center of the 80m dipole as high as I could and when I ran out of roof space I just followed the edge of the roof line around the house. I was able to tune it flat and have a 2:1 bandwidth of about 270k with 14 gauge wire. It still radiated straight up because it was so low. Hamsticks have a place, but you might be better off saving them for your emergency/portable kit.
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KE3HO
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« Reply #12 on: November 02, 2006, 01:36:58 PM »

OK, I really am not trying to be an a**hole. I am completely serious. An 80M dipole tuned with a tuner will be about as effective as a dummy load. The dummy load has the advantage that it does not require a tuner. A 60W light bulb (incandescent) at the end of a piece of coax would be a better antenna.

For that matter, someone sells (for a lot of money) an antenna that is nothing more than a 50 ohm resistor across the coax with a metal pole attached. Even that would be better than an 80M hamstick dipole.

Are you really so restricted on space that this is your only choice?

73 - Jim
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KE3HO
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« Reply #13 on: November 02, 2006, 01:54:17 PM »

Sorry, that first line should have said "an 80M hamstick dipole....."
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K4SAV
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« Reply #14 on: November 03, 2006, 10:32:29 AM »

I know this is not a very efficient antenna, but I didn't really have a good guess as to what the number was.  I do now, thanks to the SWR measurements you mentioned.

If there was no resistive loss, the impedance of this antenna would be about 0.9 ohms in free space. I placing a dipole the same shape as a two hamsticks at a height of 10 feet and added a lot of lossy coils along part of their length to obtain resonance on 80 meters and with the SWR points you mentioned. The efficiency of that antenna in free space was 1.6%, so ground losses are not included in that number.

Antenna gain straight up for 10 ft height was calculated by EZNEC at -21 dBi. At 30 feet height it is about -15 dBi.

The Hamstick folks rate this antenna as having a 2 to 1 SWR bandwidth of 10 KHz on 80 meters, so your measurements seem correct.

Jerry, K4SAV
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