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Author Topic: horizontally mountedmobile screwdriver antenna  (Read 5261 times)
WA5VSK
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Posts: 35




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« on: July 10, 2012, 11:51:23 PM »

I am curious if anyone has attempted to mount a screwdriver antenna (any brand) horizontally with a vertical whip.  If so what results did you get?

I have discussed the idea with other hams that run mobile as well as Robert at Tarheel Antennas but thought I would canvass the readers of this forum for their thoughts.

Thanks.

73 and good health to you and yours,

Bob
WA5VSK
« Last Edit: July 10, 2012, 11:55:40 PM by WA5VSK » Logged
M6GOM
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Posts: 884




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« Reply #1 on: July 11, 2012, 03:18:08 AM »

I would expect it would perform terribly unless you can get it up high enough.
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WX7G
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Posts: 5948




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« Reply #2 on: July 11, 2012, 06:12:54 AM »

This will be similar to bumper mounting a mobile antenna on a van. The portion of the antenna below the vehicle roof line does not radiate and so the antenna effective length is that of the portion above the roof line.

Your antenna will work as well as a base loaded whip and this can be modeled easily in NEC.
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WB6BYU
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« Reply #3 on: July 11, 2012, 12:53:55 PM »

I've used a horizontally mounted mobile antenna, though it was an old Webster Bandspanner
(manually tuned) rather than the screwdriver type.  However, that was sticking out a
second story window and fed against the window frames for a ground rather than an
actual mobile installation.

While it probably could make some contacts, I see two issues with such an approach...

First, as WX7G mentioned, the efficiency will be low.  The portion of the antenna between
the loading coil and the feedpoint makes an important contribution to the radiation
resistance when installed in the normal manner, but won't when it is horizontal at a low
height above ground and/or the surface of the vehicle.

Second, in most such screwdriver antennas that I'm familiar with the whip rotates as the
antenna is tuned.  That makes it difficult to use a bent whip if you want to keep it vertical.


If you just want an automated base-loaded antenna, use an auto-tuner to load the whip
instead of the screwdriver mechanism.  Or use a standard rotary inductor as a base loading
coil and drive it with a reversible motor.  You don't need the strength or complexity of a
screwdriver antenna to do it.
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AB4D
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« Reply #4 on: July 11, 2012, 04:14:25 PM »

I think it depends on the specific installation.  You didn't really give much information regarding the type of vehicle or how/where on the vehicle you plan to install the antenna in a horizontal position.

If you plan to install it close to a metallic surface, then you'll have performance issues  However, if the close horizontal surfaces are made of fiberglass or other non-metallic surface. The performance may suffer some, but not as bad if mounted close to metal.  With that being said, I would only consider trying one of the fixed coil antennas, such as the GS antenna GS-3 or GS-4. A screwdriver antenna where the coil moves up and down, is not going to be a candidate for the installation you are considering.

Gary who makes the GS antenna, has a photo on his site of a GS antenna mounted horizontally under a camper shell with the whip mounted vertically outside.  http://www.gs-mfg.com/pages/Photos.html

I asked Gary about that installation, he indicated the degradation was minimal maybe 6 db or less.

73 
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WA5VSK
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Posts: 35




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« Reply #5 on: July 11, 2012, 10:25:32 PM »

Many thanks to all who responded.

I have run mobile setups off and on since I was 15 yrs old, 44 years ago. I ran Hustler bumper mounts back then and did quite well using Heathkit gear.  I have had the mini Tarheel on a 2003 Avalance and now on a 2010 Avalanche.  One was mounted on the edge of the hood on the driver's side. On the 2010 it was mounted at the back corner of the passengers side bed of the truck like WA9PIE did.  He used to and still might have pics on his website of that install. I won't go into all the grounding, etc., suffice it to say it went from minimal to just about everything grounded that could be grounded. Performance was abysmal. I gave it to a friend to try on his Tahoe (1999 I think) he was not impressed either using a bumper mount.  Put it on the hatchback of a friends VW car(don't remember the model) and it seemed to work well on a very short trial basis with no grounding.  Trial was too short to really draw any conclusions.  Although he has one of his own and loves it.

Discussion at our club one night during a General Class lession we were giving turned to mobile antennas. Some of the fellows felt the coil did not radiate significantly but the whip was what was important.  I discussed this with Robert at Tarheel Antennas and he said there would be some radiation from the coil and the antenna was designed to be mounted vertically.  He also indicated the vertical mounting would keep water out of the coil chamber. I am not sure mounting it horizontally creates a problem with water getting in anymore than mounting it vertically would.

I have acquired a 34 ft RV and Judge,my one year old, 140 lb, English Mastiff that I rescued, and myself are about to embark on a 5,000 mile road trip. I don't have time to play around with the antenna before the trip but was wondering how it might act on the rubber roof of the RV  using an electric motor to raise and lower it. I was also thinking of trying it horizontally on the top of the Avalanche cab.  One thing is for sure. It can't perform any worse than it has so far.  Tarheel makes a good screwdriver antenna but this one just won't work for me.

Anyone that has been mobile for any length of time will find out about the idiosyncrasies of mobile operation. What works for one rig, vehicle or person won't necessarily work for others.  There are many great websites with a wealth of information on mobile operation K0BG being just one of them.

If I get industrious and try different installations I will post the data I get here.  It will be several months though before time permits that. Meanwhile I will load the Hustler coils as Judge and I cruise the highways.

Thanks again and 73.

Bob
WA5VSK

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WX7G
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« Reply #6 on: July 12, 2012, 07:42:12 AM »

I like your plan to mount it on the roof of the RV using an electric lift-and-lay to raise it. It needs to work against something and one turned radial per band would do the job. If you mount it near one corner of the 34' RV you have enough room to run radials for 40-10 meters. A current choke at the antenna will help reduce RF returning through the vehicle/RV metal and up the outside of the coaxial cable. The MFJ-915 RF Isolator might be a good one.

With one radial per band the radials will radiate. If you don't want radiating radials, two radials per band mounted opposite one another will work. In that case the place to mount the antenna is 17' from one end of the RV. There is then room to run straight radials for 20-10 meters. Radials for 40 meters can be bent to fit.

The entire length of the Tarheel radiates. The coil radiates due to the current along its axis; that is, it radials no more and no less than a piece of wire having the same linear length. If the coil length is 12" end-to-end it radiates like a 12" length of wire.
« Last Edit: July 12, 2012, 08:15:55 AM by WX7G » Logged
WX7G
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Posts: 5948




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« Reply #7 on: July 12, 2012, 09:58:08 AM »

For 80 meter operation a loaded radial can be used. A 30' wire with a 24 uH inductor will work.
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WX7G
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Posts: 5948




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« Reply #8 on: July 12, 2012, 11:14:24 AM »

I ran an EZNEC model and the two radial per band configuration results in much better low angle radiation. The downside is that the input impedance is quite low. That can be taken care of with a 1:4 matching transformer or a variable matching transformer at the antenna.

The choices are the DX Engineering DXE-MM-1 at $110 or the MFJ-901. I have a DXE-MM-1 and like it but I recommend the MFJ-901 because it has a wider impedance range.
« Last Edit: July 12, 2012, 11:20:02 AM by WX7G » Logged
M6GOM
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Posts: 884




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« Reply #9 on: July 12, 2012, 01:16:41 PM »

Tarheel makes a good screwdriver antenna but this one just won't work for me.


I have the Little Tarheel II. I swapped out the pathetically short whip it comes with and changed for a 72" one. You lose 6m but the performance increase on all the other bands is well worth it, the difference was like night and day.

To give you some idea of how well it works, I'm in the UK and use 100W. Last Wednesday on the way home from my amateur club meet on 20m I worked a station in Rio de Janiero and also N1DZ, Lincoln RI. Distance to N1DZ was approximately 3200 miles. The furthest I've worked /M with that antenna is just over 8000 miles on 10m to the Falkland Islands.

I regularly work the USA on a weekly basis with it.
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W5DXP
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« Reply #10 on: July 12, 2012, 05:36:59 PM »

Some of the fellows felt the coil did not radiate significantly but the whip was what was important.

The highest degree of radiation comes from the straight section below the coil because that is the highest current section that is a straight radiator. If you turn that part horizontal and at a low height, you have an NVIS antenna with extreme chassis/ground losses. It is not a good idea.
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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.
WA5VSK
Member

Posts: 35




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« Reply #11 on: July 12, 2012, 08:57:06 PM »

Again, many varied views and very good comments on the subject.  I thank each and everyone for their thoughts.

73 and good health to all,

Bob
WA5VSK
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WX7G
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Posts: 5948




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« Reply #12 on: July 13, 2012, 07:00:05 AM »

I forgot to mention that if you isolate the feedline at the Tarheel the control cable also needs to be isolated. And with more than the single bead that Tarheel supplies. The coaxial cable and the control cable can be routed through the same ferrite core(s) to obtain the lowest common-mode current on the coax and control cable.

Of course the Tarheel can be mounted at the RV roofline without isolation. The RV frame (if metal), the wiring in it, the RV chassis and the truck become part of the antenna. The "antenna" length is then the distance from the bottom of the RV to the top of the Tarheel whip. This can be a good thing except it could create a high input impedance, although resonant, on the higher HF bands. The antenna becomes an off center fed vertical dipole. You'll just have to try it out. I think between the Tarheel ability to be tuned to resonance and by using a tuner to transform the high or low resistance to 50 ohms it will work well. The internal tuner in the radio might have enough range.

The suggestion by M6GOM to use a longer whip is a good one. On my Tarheel 200HP mounted in the yard I have a 3', 6', 9' and a 24' whip to use depending on the time of day and the band.

« Last Edit: July 13, 2012, 07:02:17 AM by WX7G » Logged
W5DXP
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« Reply #13 on: July 14, 2012, 06:18:27 AM »

The suggestion by M6GOM to use a longer whip is a good one.

Another suggestion is to install a large top hat at the legal height limit for mobile antennas. Here's a picture of such.

http://www.w5dxp.com/shootout.htm

The top hat increases the average current in the straight section under the coil thus increasing radiation efficiency. Note that the symmetrical horizontal top hat does not radiate appreciably. In general, anything above the top of the coil on a legal height mobile antenna is a poor radiator, i.e. a base-loaded system is only half as efficient as a comparable center-loaded system.
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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.
M6GOM
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Posts: 884




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« Reply #14 on: July 15, 2012, 07:29:57 AM »

Thats all well and good if you can rig up such a support but in the UK we drive normal vehicles, not oversized tanks.  Grin

You couldn't drive that down the road at 70MPH without a lot of support.
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