Screwdriver antenna,miracle or what?

[RolandSWL]

Hi All,

 Sorry if this has been beaten to death already. I have been reading FABULOUS claims for screwdriver-type antennas and wonder how much of these claims are true.
 How can  43" coaxial tubes of coils cover 160 thru 6 meters, withstand 1500watts (P.E.P.), and communicate with stations from Canada to Kashmir ?
 If these claims are even half true, it could be the answer for folks with small lots or only balconies.
What is the real skinny?

Roland...............................

[WA2SQQ]

It's only a remotely tuned antenna. I can load a garbage can up with a tuner which fools the transmitter into thinking it's loading into a good antenna. It lacks the large capture area of a real antenna and it's extremely narrow banded. Will you be able to make contacts - absolutely - as good as a resonant wire antenna - no way. In cases where you have antenna restrictions, any antenna is better than no antenna. It's all about band conditions and how good of a counterpoise you have.

[M0VRF]

Hmm, sounds like B.S. to me.

Be assured whatever claims are being made are for the gullible to consume.

Sure you can TX on a wet piece of string and even I've had confirmed QSO's from U.K. (100W on 20m with a horiz dipole at 10') to New Zealand mobile & to Hawaii (20W on 17m with 3/8 vert) with what I consider to be extremely compromised antennas and low(ish) power, but for any antenna to be resonant on all the bands quoted and radiate efficiently is asking too much.

Oh, I've actually looked at it now, WoW a tiny antenna with a MASSIVE loading coil on it, Brilliant..............Who thought of that?

I suppose it may look OK on those oversized cars you all drive!

Just think of it as a 10' antenna with a 50 Ohm resistor to ground!

 

Stretchy.

[K1JJ]

Hi Roland -

No miracle - just another compromise antenna design. The advantage is using a  remotely controlled coil - with the disadvantage of slightly larger coil losses compared to an optimized Q Bug Catcher style coil.

Back in the mid 90's a group of us AMers spent a lot of time experimenting, building and testing mobile antennas, mainly for 75M.  We built coils of all sizes and added whips that were to the limits of mobile use before hitting bridges, etc.  We once had eleven 75M mobiles on the air coming back from Hosstraders flea mkt one Saturday night.

I personally tried all kinds of large whips, some large copper poles with huge coils. It was crazy to look at. One guy wound a 5 gallon PVC can with 3/8" copper tubing as the coil and mounted it in the back of his pickup truck with an 11' whip.

We set up comparison tests in a large empty parking with a remote station listening several miles away taking s-meter readings as we tried everything under the sun.

The bottom line is that on the lower bands (especially)  we are severely limited by the lossy ground plane of any vehicle.  No matter how big the whip is (even a 60' mast) and a great copper tubing coil, the losses in the path to ground act like a big resistor absorbing the majority of power.

So, unless the coil and whip are very poor, like using a small Hamstick or whatever, there is little difference once you get to a certain whip/ coil quality level.  Signals vary very little in db at a remote station once we get to a certain level due to the ground losses overwhelming the system..

The best antenna we tested was the homebrew copper tubing coil and 11' whip. The coil needs to be near the center of the whip for best performance and Q. This was followed by the Bug Catcher style antennas which were down maybe - 1 dB.  The Screwdriver faired well at maybe -2dB down.  If the overall installation is good, these three antennas all worked quite well.

The BIGGEST difference was based on how big the vehicle was. The trucks did MUCH better (by 2-3 dB) compared to the smaller cars, owing it all to the bigger groundplane to Earth.   One of the guys actually attached 60' fine wires to his car bumper and drove down the highway... :-)

All in all, I would pick the Screwdriver.   That's because a homebrew or Bugcatcher is critical to adjust on a given frequency and needs fine tuning when in the rain, gets covered in salt, etc... and cannot be adjusted on any band remotely. I would go with a quality remote-controlled Screwdriver and eat the 1-2 dB loss.  The smaller coil is not perfect, but will do. Even though a couple dB is precious when mobile, the overall difference is so small over a full size homebrew or Bugcatcher, I would opt for the ScrewDriver design due to laziness... Nothing like tooling down the highway and adjusting your swr on the fly when QSYing or getting into rain.

BTW, the best 75M mobile antenna system going would be a 55' tractor trailor with a Bugcatcher style antenna - and followed by a screwdriver for convenience. That would be even better on 40M and up.

There are probably poor quality/ cheap screwdriver designs out there, so check them out first.  Look for the biggest diameter coil and largest gauge coil wire - with the most robust coil contactor..

As a baseline, I once ran my very best mobile installation against my 75M, 60' high full size dipole as a comparison. There was always 12-15dB difference in favor of the dipole. We are talking vertical vs: horizontal polarization, granted, but I did the tests when conditions were perfect for the vertical, like at sunset.  So expect to be down at least  ~ -15dB when mobile no matter what you use. It will never be better than -15 dB on 75M against a home station. If the installation uses a tiny car and lossy coil/whip, it will be closer to -20-22dB down which is a serious disadvantage on 75M.   The higher bands will be proportionally better performance. On 160M it may be as bad as -30 dB down as the ground losses mount and the whip losses increase due to the low impedance because of short size..


Tom, K1JJ

[AB2EZ]

Building on what Tom and the others have said in their replies:

Antenna theory and antenna behavior can be very mysterious and complicated... which leaves lot's of room for the "Volkswagen" approach to the manufacturer's specifications.

Even if the antenna's physical components (e.g. the loading in coil in this case) are lossless; and even if the antenna presents a perfect 50 ohm resistive load to the transmitter... that does not imply that the antenna is an efficient "radiator".

It is true that in the above, ideal case, all of the power that leaves the transmitter will leave the antenna.

But, the part that many people have difficulty understanding (because it is complicated) is that:

Of the total power that leaves the antenna, only a portion is coupled to electromagnetic "modes" that radiate away from the antenna (decaying in field strength as 1/r).

The remaining portion that leaves the antenna couples to non-radiating electromagnetic modes that do not radiate away from the antenna. Those modes decay exponentially... and are responsible for resistive losses (heating) in nearby objects (like the surrounding ground, nearby lossy conductors, etc.)

When an antenna is electrically short (i.e. compared to 0.5 wavelengths), a smaller (maybe much smaller) portion of the total power that leaves the antenna couples into radiating modes. A larger portion of the power that leaves the antenna couples into non-radiating modes that heat up the surroundings.

It is also the case that an electrically short antenna has a higher (perhaps much higher) near field strength... because the near field consists of the power coupled into the radiating modes + the power coupled into the non-radiating modes.

It is also true that neighboring conductors (like the metal body of a car) can act like part of the antenna... and therefore, a physically short antenna might be transformed into a somewhat larger (or even much larger) antenna by the presence of nearby conductors.

[As an example, note that the reflector of a dish antenna is not physically connected to the antenna's driven feed horn... but the effective size of the antenna is that of the reflecting dish]

Stu

[WB5IRI]

What they said.

However, I have used any number of antennas on various vehicles over the past 40 years -- Hustler (a real dummy load on 75), bug catcher, screw driver homebrew, screw driver commercial, and some experimental designs that mostly just looked weird and didn't work well at all. The best was the bug catcher, but I have an 80-mile commute each way every day, and I found that if the band changed and signals declined, I simply was NOT going to pull over on the interstate and change taps on a coil. Next best was the HiQ brand of screwdriver -- http://www.hiqantennas.com/ -- with the 4-inch coil. Almost as good as my bug catcher, but I could change bands without even having to come off cruise control. Less than 15 seconds or so to change bands and tune for lowest SWR, and I would be back on the air.

Any mobile antenna is a compromise, and you just aren't going to compete on the same level as someone who has a full-size resonant antenna. But you will get out, people will hear you, and, surprisingly enough, you will break pileups and work DX from your vehicle. Don't let those who tell you how lousy mobile antennas are scare you away from having a ball going mobile.

I have talked to guys who lived in apartments and clamped their screwdrivers to their balcony railings or to their carports. As good as a dipole? No. Did their signals get out and did they work lots of stations? You bet.

Best of luck,

Doug

[K1JJ]

Hi Doug,

Those look like quite strapping antennas. Amazing that they are able to tune from 1.8 - 30Mhz in less than a second.

Beefy and prices are stiff, which is what you'd expect for the high quality.

T

[KD6VXI]

HiQ is a great antenna, for a mobile.

There is also a dual version.   Two screwdrivers,  a la a dipole.   Probably the best setup you'll find,  money is no object.

I loved mine.   I ran it on a one ton dodge,  with a legal limit +++ mobile amp,  kept at approximately 1.5 kw pep.

My mobile,  sitting outside my house on 75, had almost an equivalent signal as my home station.

Screwdriver and legal limit mobile vs full sized 160 inv v,  ladder line fed,  Johnson mini my and a hundred watts pep.

--Shane
KD6VXI

[K1JJ]

My mobile,  sitting outside my house on 75, had almost an equivalent signal as my home station.

Screwdriver and legal limit mobile vs full sized 160 inv v,  ladder line fed,  Johnson mini my and a hundred watts pep.

--Shane
KD6VXI

Yep, that makes sense. You were seeing about  12 dB difference (1500W vs: 100W power difference)  in favor of the inv v on 75M.  

Most of the time the difference will be more because the locals favor a horizontal polarization on 75M.  When the conditions are changing rapidly, like at sunset, we find a mobile vertical can come up in strength, though cannot ever recover from the ground losses.

On 40-10M, especially 20-10M,  if the vehicle is large enough, a well-tuned and efficient mobile whip can work almost as well as a full size one with ground radials.

T

[WB4AM]

Hello,

I am certainly a believer of the screwdriver antenna.  I have talked over to Spain, England, Italy and other locations but they were closer in.  I first used the High Sierra HS-1500.  (10-80 meter) I couldn't believe how well it worked.  Most of the time I was using 3 to 4 hundred watts, but none the less I was making contacts.  

Later just for the heck of buying something when I used to go to Dayton, I bought a Tarheel screwdriver.

I forget the model.  It had a 4 in it?  Anyway they don't make it anymore.  It was 10-160 meter.  It was built a little bigger then the HS-1500.  I have always thought it performed better then the High Sierra.  However I was on my way home late one weekend and I heard some local AMers on 160 and I knew where one or two of them lived.  I wasn't to far from them and they never came back to me when I called out!  

So as far as 160 meter is concern, forget it!

But from 10 through 80, I won't use anything but a screwdriver like antenna.

I don't know if this helps, but I also attach wires from the ground side of the base of the antenna and run it around and under through the truck frame.  Actually to date I have been using cat5e wire.  Since it has 8 conductors, it gives me 8 so called radials if you will.  Not really but...In hopes that it makes up the other half of the antenna. (Dipole)

Ken

[W3NE]

In cases where you have antenna restrictions, any antenna is better than no antenna. It's all about band conditions and how good of a counterpoise you have.

That quote fits my situation exactly, as do Roland's final comments. I live in a third-floor apartment of a "Life Care" retirement facility where uptight management refused to allow me to run a 170 ft. wire from the balcony to a tree. They didn't say I couldn't put something on my own balcony, however, so I began by installing a Pixel Technologies Magnetic Loop on a rotating mast for reception. Several Tx antennas were tried, including a screwdriver antenna mounted at 45 degrees per recommendations from both HiQ and Tarheel. No luck; the ground plane was inadequate and varied from band-to-band, as might be expected. I don't like unbalanced antennas anyway, so another SD antenna (Tarheel 100A) with 6 ft. whip was obtained and mounted in a dipole configuration with the first one. HiQ would have been preferred but the cost was almost twice what my dipole cost. A 1:1 balun is used between the dipole and coax to the shack.

The two antennas are mounted on a high density polymer channel at the end of a 2"x4" rectangular aluminum tube with brackets holding the tube on a balcony railing to extend the dipole about as far out as I think it can go before people begin complaining. This works great (for what it is). Each side is tuned with a separate Ameritran remote control with 10 memories for coil positions.There is interaction between the two sides of the dipole so a TenTec graphic SWR analyzer is used to tune for 1:1 SWR and 50 ohms. A chart was made for frequency increments on 75, 40 and 20 corresponding to antenna bandwidths on those bands so I can pre-tune for rapid QSY. Low antenna efficiency is partially compensated by an SB220 linear amp.

An RF control center enables switching between normal operation and the SWR display, as well as Barefoot/QRO, and antenna/dummy load, with three toggle switches controlling coaxial relays and an interlock to prevent a dumb operator from transmitting into the TenTec analyzer. SWR and power are monitored simultaneously at exciter output and linear output with two Elecraft W2 SWR detectors located in the control center. There are no coax cables to juggle and all controls are at one location, a bow to the Op's physical condition. This would not be possible without availability of reasonably good screwdriver antennas. It is not an ideal situation but it works under the circumstances.

Local noise in this place is horrendous, S9+10 on 75 and S7 on 40 on good days, even at 3 a.m. A transceiver with excellent DSP helps, but noise is noise!  It is the greatest impediment to enjoyable operation but being able to have some QSOs is better than none. A "noise canceller" hasn't been tried yet but that might help. One final point: A chance was taken that water would drain from the antennas but it takes a long time and in winter it freezes before it all gets out. Material is on hand to tilt each side of the dipole by ten or fifteen degrees, which should take care of that. A photo of the present installation is attached.

Bob - NE

[WB5IRI]

I love it! Where there is a will, signals will radiate!

[RolandSWL]

A big thank you to all. This information is much appreciated.

Roland............

[WB2EMS]

I'm really sold on the screwdriver design for mobile, and even for some sorts of portable or non-traditional mobiling like marine mobile. I've had a couple of the flavors that push the coil up out of the lower pipe, but now have one of the Hi-Q models, a 5 inch Pirhana. I like his sealed design better - it doesn't get the corrosion from winter weather exposure that I had with the previous flavors. And I think it approximates the bug catcher better, with the big Hi-Q coil, although it does short the turns which is supposed to hurt the Q, but so do all bug catchers and they still come out on top.

When I switched on my current vehicle from one of the shorty HiSierra models to the Hi-Q, I did some before and after testing by setting up my car about 100 yards away from my HF vertical antenna and running about 5 watts carrier and making measurements with my Flex5000 as spectrum analyzer. I did that test with both antennas and the same top whip, and on 75 meters, the hi-Q was 20db (!) better signal than the other antenna. Some of that was due to the shorter antenna having the coil closer to the body, but 20 db is a lot and I think at least 10 db is due to the better design. OTOH, I had to add a support bracket to handle the extra weight of the new antenna up high. And people think I'm one of those google cars with the cameras on it when I drive around. 

Now all I need is a big turbo diesel pickup truck for a better groundplane than the Golf TDI 

[K1JJ]

When I switched on my current vehicle from one of the shorty HiSierra models to the Hi-Q, I did some before and after testing by setting up my car about 100 yards away from my HF vertical antenna and running about 5 watts carrier and making measurements with my Flex5000 as spectrum analyzer. I did that test with both antennas and the same top whip, and on 75 meters, the hi-Q was 20db (!) better signal than the other antenna.

Hi Kevin -

Interesting about the sealed design. That would be my choice too.

BTW, how short was the HiSierra Shorty compared to the HiQ for the test?

Yes I'd agree.... 20dB difference is a lot - wow.    20 dB is like one antenna putting out 1000 watts and the other putting out 10 watts.  I'm guessing the HiSierra had a problem of some kind?  Grounding, tuning, severe interaction, bad connection, etc.  

As a control test, mount them one at a time on the ground in the clear with some radials with each tuned to 1:1 swr and try the test again.  I'm guessing the difference will be 3-4 dB max depending on how similar the configurations are.

T

[flintstone mop]

There is a very interesting article in this month's QST about antenna myths and antenna height.
It's like a crap shoot when dealing with MaNature. SHE determines whether the angle of radiation is high or low.
You can have a blast with DX on 10M, when the band is hopping, with an antenna 20 feet high from the ground.

I still think back to a wonderful analysis from our Tom, K1JJ and his experimentation with his massive 40M yagis.

http://amfone.net/Amforum/index.php?topic=21151.0

One of the finest threads ever on AMFONE!!

[Steve - K4HX]

Kevin - you were too close for your tests. At a distance of a mile or so, you'll likely see less difference.

Bob - Very nice set up. Where there is a will there is a way. What is attached to the rotor?

[W3NE]

Thanks, Steve. The rotator is for a magnetic loop receive-only antenna, mounted above on a 20 ft. mast with a secondary bearing on the balcony column. I wanted the rotator where I could get to it for service if needed. Gnd view attached.

Not on AM from here yet, but WB3LGG is completing major restoration of my Ranger. This is a far cry from former QTH, but . . .

Bob - NE

[n1zpy]

HiQ is a great antenna, for a mobile.

There is also a dual version.   Two screwdrivers,  a la a dipole.   Probably the best setup you'll find,  money is no object.

I loved mine.   I ran it on a one ton dodge,  with a legal limit +++ mobile amp,  kept at approximately 1.5 kw pep.

My mobile,  sitting outside my house on 75, had almost an equivalent signal as my home station.

Screwdriver and legal limit mobile vs full sized 160 inv v,  ladder line fed,  Johnson mini my and a hundred watts pep.

--Shane
KD6VXI

Where on your one ton did you have the antenna mounted?

[KD6VXI]

I used a CB antenna mirror mount, and took a bit large enough for an SO239 bulkhead and enlarged the hole for the 3/8x24 stud.

Mounted that mount center of the cab, on the bed.  I wasn't totally happy with this, so I elevated it with a piece of half inch rigid conduit so the coil was almost completely above the bed.

Because of the power output, I wanted the feed point up as high as possible.  Probably a moot point since I basically kept current maxima at the same level.

It DID work, though.

--Shane
KD6VXI

[n1zpy]

I only plan on running 1kw.  My biggest question is would inside the bed be better having more metal under the antenna or outside the bed?  Mount will be in the left rear corner.

[KD6VXI]

If I had to do it all over again, I'd mount it directly on the roof!  Up as high as possible and out of the way.

At the time, my better half was NOT having it!

In the clear and out of the way is best.  Whatever you do, DON'T get one of the trailer hitch mounts.

--Shane
KD6VXI

[n1zpy]

After much thought I mounted it on the right side just ahead of the wheel well inside the bed.  We chose that side because the stack is already on that side.  At over seven feet to the top of the roof that was ruled out lol.

[W2PFY]

It also helps if you can run about 250 watts from your mobile. Not many people do but those who I have worked consistently had a louder signal than those who did not.

[KD6VXI]

He's planning on a kilowatt.

I ran an amp that was capable of about 600 watts carrier and a hundred pct mod, but kept it at 1.5kw since I was scared about the antenna.

Two alternators for the amp.  Lots of room with a Cummins!

--Shane
KD6VXI