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Author Topic: Screwdriver dipole antennas  (Read 721 times)
AG4DG
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« on: May 16, 2003, 11:54:07 AM »

Does anyone here have a Screwdriver dipole antenna?  Has anyone here seen one?  (One of you mentioned that High Sierra sells a Screwdriver dipole, but I couldn't find it on the web site.)

The Hamstick dipole antenna is quite popular.  It's more efficient than a Hamstick monopole antenna, because the ground loss is taken out of the equation.  Of course, the coil losses remain.

I think a Screwdriver dipole antenna would be even better than a Hamstick dipole.  I realize that the good mobile antennas like the Screwdrivers and Bugcatchers are heavier and require sturdier supports than Hamsticks, but I am sure there is a way to mount them in a dipole configuration.

The advantages of a Screwdriver dipole over a Hamstick dipole are:
1.  Multiband resonance available
2.  Even more efficient due to reduced coil loss
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AD7DB
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« Reply #1 on: May 16, 2003, 12:30:54 PM »

Well, this is what ham experimentation is all about, isn't it?

At least two concerns. You have to make sure that both sides are properly tuned. Second, the weight of these screwdrivers, because they're horizontal, means a really sturdy center mount, as well as added weight overall, maybe too much for most mobile installations. Would this stand up to doing 65 on the highway?
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K5LXP
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« Reply #2 on: May 16, 2003, 12:54:26 PM »

Ground loss is only out of the equation as long as you have your screwdriver dipole up a half-wave or so.  If you're running a 40M configuration you're up 70 feet.  If you've got the means to haul a screwdriver dipole up 70 feet you'd be way better off hanging an inverted V or dipole instead.

At $300-$500 apiece for the screwdrivers, you have one awfully expensive compromise antenna.  But it would certainly work as you describe.  The SteppIr dipole/beam is a better way to go if you're looking for a remotely tuneable antenna.  Since they use no loading they will be more efficient.

Mark K5LXP
Albuquerque, NM
k5lxp@arrl.net
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AG4DG
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« Reply #3 on: May 16, 2003, 01:26:01 PM »

You have your points, but I consider a Screwdriver dipole to be a substitute for Hamstick dipoles, not a regular mobile setup.  I don't think anyone uses a Hamstick dipole mobile, as the car body provides capacitance to ground, which is close at hand.  I thought that Hamstick dipoles are for upstairs apartments due to their space efficiency and lack of access to ground.  (Basically, monopole antennas belong on the ground or mobile.  Dipole antennas belong up high.)

Also, there is no law or regulation saying that dipoles must be horizontal.  They could be diagonal or vertical, too.

Having said all this, there is still the challenge of supporting a heavy Screwdriver (or Bugcatcher) antenna in dipole form.  Has anyone done such an installation?
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K5LXP
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« Reply #4 on: May 16, 2003, 02:19:56 PM »

It would seem the mount for such an antenna would depend a lot on the actual antenna chosen.  For instance, the Yaesu ATAS antenna uses a PL-259 type base.  It would be a simple matter to make a suitable bracket to hold two SO-239's, connect the coax and screw the antennas on.  Other HF screwdrivers use the standard 3/8-24 stud, a bracket for which could be just as easily constructed.  The more important issue you need to address is water ingress.  Most HF screwdrivers expect to be mounted vertically.  If mounted any other way water could collect in the mechanism and cause problems.  I don't think you're going to see too many examples of this idea due to the expense to performance ratio.  Not many hams are going to spring close to $1K for an antenna setup that is arguably no better than a random wire and a tuner (quite likely less so).  One way to know for sure- put one together and write an eHam article about it.

Mark K5LXP
Albuquerque, NM
k5lxp@arrl.net
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K0BG
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« Reply #5 on: May 16, 2003, 02:37:12 PM »

The biggest issue I think is tuning it. Instead of spending all that money for two screwdrivers (unless you already own them) is to look at the IR dipole. Costly yes, but less than two SDs for sure.

Alan, KØBG
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KD5FOY
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« Reply #6 on: May 16, 2003, 04:16:54 PM »

look at Steppir Antennas.  expensive antennas but
they sure do work.  they make beams, rotatable dipoles
and verticals.  i've used their products for over
a year and have nothing but praise.  no connection
to them except for a very satisifed customer.

http://www.steppir.com/

larry
kd5foy
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W0FM
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« Reply #7 on: May 16, 2003, 05:37:18 PM »

Actually, many years ago, I sent a note to Don Johnson (father of the screwdriver antenna) and Lew McCoy (famous antenna guru...now SK) and asked about the possiblity of using two screwdriver antennas back-to-back as a dipole.  Interestingly, Lew said that the two haves of the dipole would NOT have to be synchronized to exactly the same length.  That surprised me, but he said that there would be some benefit to being able to tune both halves independently for lowest SWR.

Now High Sierra and, I believe, some others offer screwdriver dipoles.  They ARE large and heavy looking for sure.  I agree that the SteppIR tunable dipole is a better (albeit fairly expensive)approach.

Good luck.

Terry, WØFM
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N6AJR
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« Reply #8 on: May 16, 2003, 09:32:33 PM »

Yes it will work, but for the Money you can do much better. A rotatable dipole for 40 to 10 is a great start and only a couple hundred bucks ( cushcraft D4).
A nice triband beam is a bit more, and a pair or two of ham sticks is under a hundred dollars.

 You can put up a fan dipole ( 1 coax to multiple inverted v's for each freq you work ) for the cost of the wire and a tall pole in the center. You can either drop the shorter dipoles under the long ones or "star" them with the longest north and south, the next longest run east and west etc.  Just tie some 3/8 nylon rope to the end of the dipoles to reach the eve or fence or whatever.  

Using 2 screwdrivers would work but poorly, and be very expensive for the performance you get.  A 5 or 6 band verticle on a 30 foot mast with two sets of four 66 long foot guy/radials  mite be an option .  

You can even make a switchable beam for 40 by putting up a 50 foot mast , cut 5 dipoles for 40 (66 feet +-) that is 33 feet each side with the coax in the center to the box)

Run them as slopers in different directions from each other (one sloping NNW, one NNE, one West , one south, 1 east , or which ever way is your favorites.).  You use one of those ameritron remote switch boxes at the bottom ( put it in a water proof box) of the pole.  You use equal length coax leads to the box at the base of the pole.  When you select one of the 5 buttons, say, the south facing dipole as active it becomes active and the other 4 ( the box grounds the other connectors at the box) look longer due to the shorted "stub" and act like reflectors. ( This one is compliments of AC4CD)

Bingo, a great "beam  on 40.  Try other things before dropping $400 to $600 on a pair of screwdrivers and associated stuff to make it work.  Folks this is Ham radio, not rocket since.

 hope this helps  tom N6AJR
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