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Author Topic: Screwdriver dipole  (Read 8044 times)
KE5KDT
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Posts: 47




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« on: August 21, 2013, 05:54:54 PM »

I have reviewed some previous eham posts on using two screwdriver antennas but didn't really get the question answered yet.
I have two Yeasu ATAS 100 screwdriver antennas I want to join for a tunable dipole.  Yes, I understand this will not work as well as a thousand other antennas, but I want to try it anyway for the fun of it.  I will put it on a mast about 8 to 10 feet tall in the yard.  This is not for mobile use.
Yaesu powers the antenna motor and carries the RF on just the coax.  No separate power lead.
My question is about tuning the antennas at the same time and whether the two electrically hooked together will tune properly, i.e., extend each antenna just the right amount when driven by the radio, which happens to be an FT-897D.  I also have the MFJ box which will let other radios use the ATAS antennas.
Thanks, Bob. 
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M6GOM
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« Reply #1 on: August 22, 2013, 05:00:51 AM »

First of all the radio doesn't have the capability to supply sufficient current to drive two and would see the higher current drain of two antennas as the stall current of one so would randomly reverse direction of travel so I would forget that one.

So we move to option 2, the MFJ solution or a manual solution. That will allow both antennas to be moved however you cannot guarantee that they'll move at the same speed and over a period of time a quite notable discrepancy is likely to occur. I would say that you would get a low SWR but due to the mismatch between the sides would suffer a fair bit of common mode on the feeder and/or motor controller cables.

However in your post you make the most important argument for doing it:

Quote
but I want to try it anyway for the fun of it.

Isn't that what amateur radio is all about? Try it, have fun trying it. If it works then its a bonus and if it doesn't then you've learnt something.
« Last Edit: August 22, 2013, 05:03:04 AM by M6GOM » Logged
KE5KDT
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Posts: 47




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« Reply #2 on: August 22, 2013, 05:23:01 AM »

The more I think about this I doubt it will work as I hoped.   I was going to use a coax connector in a T arrangement to hook both antennas to since the power to run the ATAS goes through the coax.  While it might drive both motors how would it sense when it was at the correct length for a particular frequency?  Normally you have the two sides of a dipole, shall we say a plus and minus as one works with the other.  In a screwdriver antenna I just have one side with the vehicle being the counterpoise that it tunes against. 

Possibly, one antenna could be run out 1/4, 1/2, 3/4, or full depending on the freq desired and then switch to the other antenna and tune to the selected frequency.  And that would seem to require a switching circuit that would let the side adjusted for the freq to be the plus and the other pre-extended one the minus, in other words I would have to reverse the coax connection on the pre-positioned one and leave it that way for transmitting. 

The dipole arrangement might work with a regular screwdriver where you take one antenna to the jacket of the coax and the other to the center lead and then hook the motor leads together so as one acts as the tuning antenna the other one extends and retracts at the same rate until it finds the match. 

Another less elegant solution might be to put three or four different band hamsticks on the counterpoise side and then just tune the screwdriver against them.

Maybe I answered my own question.

Alan, where are you when I need an answer?
Bob
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KE5KDT
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« Reply #3 on: August 22, 2013, 05:26:36 AM »

Thanks Conor for your input while I was typing mine. I believe you are correct on all your points.  I suppose I should have asked the question before I bought two ATAS 100s.  At least they  only cost me $100 each and can easily be resold for as much.

I'll give it a few days and see what else comes in. 
Cheers.  Bob
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K5LXP
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« Reply #4 on: August 22, 2013, 06:13:17 AM »

Hi Q sells a real spendy version of a screwdriver dipole, so one has to wonder how they keep the two antennas in sync.

http://www.hiqamateurradioproducts.com/Antennas/5160tad.html


Mark K5LXP
Albuquerque, NM
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KE5KDT
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« Reply #5 on: August 22, 2013, 06:33:06 AM »

Thanks Mark. I checked the site to get info on their dipole but they are temporarily shut down for an operational/management transition.  I have other screwdrivers including a Little Tarheel and a Hi-Q 4-80.  Maybe the answer is to use conventional screwdrivers.  I have a Buddipole which is what got me to thinking linking two small screwdrivers might save a lot of time taking down and resetting the clips.   Portability is not much of an issue as I have no plans to take this anywhere I can't drive.
Love Albuquerque, spent 5 years there.
Bob
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K0GGC
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Posts: 9




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« Reply #6 on: August 22, 2013, 11:29:29 AM »

Have you tried Scorpion Antennas.

They make the best screwdriver antenna of all the manufacturers in my opinion. They also have a very nice dipole dual screwdriver setup.

See this page: http://scorpionantennas.com/index.php?route=product/category&path=63
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WB6BYU
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« Reply #7 on: August 22, 2013, 12:38:29 PM »

Quote from: KE5KDT

The more I think about this I doubt it will work as I hoped.   I was going to use a coax connector in a T arrangement to hook both antennas to since the power to run the ATAS goes through the coax.  While it might drive both motors how would it sense when it was at the correct length for a particular frequency?  Normally you have the two sides of a dipole, shall we say a plus and minus as one works with the other.  In a screwdriver antenna I just have one side with the vehicle being the counterpoise that it tunes against. 



You have to connect one of your screwdrivers to the coax center (as would be the case with
plugging the coax directly to it) and the other has to connect to the coax SHIELD.  That makes
it difficult to adjust when the tuning voltage is applied between the coax center and the shield.
The T connector would allow both to tune (assuming that  the control circuit would handle
the higher current, which it might not) but would have them in the wrong RF phase.  I'm sure
there is a way to separate the RF and DC using capacitors and coils so that the second one
is connected to the center conductor for DC but to ground for RF.  (Perhaps you could use
a half wavelength of coax between one antenna and the other to provide a 180 degree phase
shift while still passing DC of the proper polarity.)


To keep the two antennas in sync it would be better to have a single motor driving both
antennas, though I don't know how feasible that is mechanically with the antennas you have.
One approach would be to make two mirror-image antennas with a common tuning shaft,
but that would require a reverse thread on one side to get them to move in unison.
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K5LXP
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« Reply #8 on: August 23, 2013, 08:01:43 PM »

I'm guessing the spendy solutions use the hall sensors in these antennas to know where each one is to keep them in sync.  Use an isolated source of DC and which antenna is "hot" and which one is "ground" wouldn't matter.

Mark K5LXP
Albuquerque, NM
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KE5KDT
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Posts: 47




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« Reply #9 on: August 24, 2013, 06:53:23 PM »

Gentlemen, I have gotten my question answered and that was the whole point.  It won't work with Yaesu "ATAS antennas that tune through the coax.

The Scorpion screwdriver dipole, that was referred, is a nice design and is easily doable with regular screwdriver antennas that have a separate set of leads to run the motor.

Thanks very much for all the inputs.
Bob

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KG4RUL
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« Reply #10 on: September 01, 2013, 11:13:30 AM »

HI Q is not accepting any orders at present http://www.hiqamateurradioproducts.com/store/

Hi Q sells a real spendy version of a screwdriver dipole, so one has to wonder how they keep the two antennas in sync.

http://www.hiqamateurradioproducts.com/Antennas/5160tad.html


Mark K5LXP
Albuquerque, NM

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K1TWH
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Posts: 103




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« Reply #11 on: September 04, 2013, 01:52:10 PM »

Bob,
     There is some merit in having a one Hamstick/band as a counterpoise.   It's only truly balance over a narrow band, but would work over a wider bandwidth because you can change resonance the ATAS100.  I've used two 40M Hamsticks and found them down about 6dB from a full size dipole (both at about 25 feet high).   Higher Q would reduce the loss (and the bandwidth).     Tom  WB1FPA
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