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Author Topic: Screwdriver antenna questions  (Read 4690 times)
AC4RD
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« on: April 22, 2011, 05:34:58 AM »


I've asked my mobile HF antenna questions here before; I've got another question (more like a vague idea) and would welcome input about it from the group.

My problem is that, at work I park in a garage with very low clearance, maybe 8 feet in places.  And I'm not willing to get out and take off or fold down an antenna every time I get to work, and then fold it back up when I leave work--that's just too much bother.

Right now I'm using a short (36") mast from DX Engineering with Hustler resonators.  This setup is low enough to let me in the garage without bother--the 54" mast won't fit.  This setup works, though I have to hear someone at S7 or better to have a reasonable chance of working him.  In other words, this antenna system is NOT a great performer.

It has occurred to me that if I got a screwdriver-type antenna, I could mount it at the highest point on my car that lets me into the garage when the antenna is fully retracted.  Then I could extend the antenna to operating position (remotely with the controls) when I leave the garage.

I'm not eager to spend the money and time installing a screwdriver unless it would also offer me some reasonable improvement in signal quality.  So what do you think?  Would a screwdriver give me a noticeable improvement over the Hustler system?  (The K0BG site seems to support that.)  Does the idea of retracting it for the garage and extending it for operation make any sense?  Thanks!  --ken ac4rd
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K5LXP
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« Reply #1 on: April 22, 2011, 06:20:23 AM »

What about the whip- do you care if that hits the garage ceiling?  Screwdrivers have whips at least four feet long, many are six feet.  The screwdrivers themselves are anywhere from 3-4 feet long.


Mark K5LXP
Albuquerque, NM
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AC4RD
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« Reply #2 on: April 22, 2011, 06:33:46 AM »

Mark, I'd rather not have the whip hitting the ceiling beams in that garage; there's at least 30 or 40 of them between me and the exit, and I don't think that many shocks, even little tiny ones, would be good every day.   So whatever I put on the car can't be more than 7 or 7.5 feet tall, even mounted down at the bumper.  :-(  Thanks!
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KG4RUL
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« Reply #3 on: April 22, 2011, 07:26:14 AM »

http://www.tarheelantennas.com/little_tarheel_ii

ANTENNA SPECIFICATIONS
Lower Mast Size -- 1 1/2"
Lower Mast Length -- 16"
Whip Length -- 32"
Total Length of Antenna in 54 MHz position -- 48"
Total Length of Antenna in 3.5 MHz position -- 54"
Frequency Coverage Continuous -- 3.5 to 54 MHz
Power Rating -- 200 watts P.E.P.
Typical SWR -- 1.5 or less
Weight -- 1.9 lbs.

http://www.tarheelantennas.com/little_tarheel_ii_photos
Look at the pictures of typical installations, on the webpage above, for inspiration.

BTW: I have owned a Lil Tarheel II for 7 years and am very satisfied with the performance.
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K0BG
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« Reply #4 on: April 23, 2011, 07:49:24 AM »

As long as you're willing to put up with the reduced performance of the Lil Tarheel (it is about the same as you have now), that will work. Just make sure you properly choke both the motor leads, and the coax cable.
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AC4RD
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« Reply #5 on: April 24, 2011, 05:15:00 AM »

Thank you, Alan!   The conclusion that the Little Tarheel II isn't significantly more efficient than my current Hustler/Hamstick setup, is what I think I understood from the "antenna efficiency" page on your website, but it's good to have it confirmed.

After looking at a lot of screwdriver websites, it appears a screwdriver really won't help with my height limitation--none of the screwdrivers have enough travel for it to be a major factor.   So I'm left with the only advantage to a screwdriver, in my own case, being the convenience of changing bands remotely, and to me, that's not worth the bother and expense.

I guess I need either to learn to live with the height restriction of parking in the low garage at work, or design a foldover system that can be operated from the driver's seat.  :-)

Thanks to all for the input and suggestions! 
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M6GOM
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« Reply #6 on: April 24, 2011, 07:15:50 AM »

Thank you, Alan!   The conclusion that the Little Tarheel II isn't significantly more efficient than my current Hustler/Hamstick setup

It is once you junk the supplied whip and swap it for a 60" or 72" one. You get an increase on 20m of several S points.
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K0BG
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« Reply #7 on: April 24, 2011, 07:29:58 AM »

Let us see... Several S points...

The actual difference in field strength measurements varies depending on the mounting methodology, but it is typically between 10 and 15 dB when compared to a decent-quality, full-sized screwdriver. Considering the accuracy of most S meters, that could be several S units, or less than one S unit; yes they are that bad!

Let us look at the whip too. The radiation efficiency of an antenna is based on all of its losses, divided into its radiation resistance (Rr). That in turn, Rr, is mostly based on the antennas overall length, and it is a square factor. However, the whip in this case is no doubt 17-7 stainless steel. The truth is, depending on the frequency (the lower it is, the worse it gets) increasing its length adds about the same amount of loss, as it gives you in the increased Rr.

There is also an issue with capacitive base loading (low mounting vs. higher on the vehicle), and since we really can't measure it directly, making a pat statement about it is subjective at best.

Where the real issue lies with the Lil Tarheel, and hamsticks is the rather low coil Q, compared to a decent screwdriver like a 200A Tarheel or perhaps a Scorpion 680. That condition is hard to overcome with just a better mounting, or longer whip.
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M6GOM
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« Reply #8 on: May 01, 2011, 07:50:56 AM »

However, the whip in this case is no doubt 17-7 stainless steel. The truth is, depending on the frequency (the lower it is, the worse it gets) increasing its length adds about the same amount of loss, as it gives you in the increased Rr.


So if I've read that right, there's no advantage over having the longer one? How come the increase in receive and TX signal? And we're not talking just a S point but going from say S3 to S7 on my TS480.

With the longer whip in, it is on par with a Diamond CP-6 antenna I used as a homebase to the point where on 20m, a station in New York some 3300 miles away could tell no difference at all. With the supplied one, it was notably worse on both TX and RX.

As for the mounting, there's only really one place on a Saloon (Sedan) and thats with a hole drilled in the middle of the roof. Cheesy

Just as an aside, is the difference between a Little Tarheel II and the 200A model that pronounced? I do a lot of HF mobile. Also thinking of a vehicle change. As putting a 200A in the middle of the roof is obviously a non-starter, where is the best place to install one on say either a sedan, a 4x4 such as a Land Rover Discovery/Toyota Landcruiser or a small van such as a Ford Transit?
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AC4RD
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« Reply #9 on: May 01, 2011, 08:16:07 AM »

Thank you, Alan!   The conclusion that the Little Tarheel II isn't significantly more efficient than my current Hustler/Hamstick setup

It is once you junk the supplied whip and swap it for a 60" or 72" one. You get an increase on 20m of several S points.

Well, a quick update to say, "Boy, are you guys ever right about that."  You, and K0BG's page on "antenna efficiency" on his mobiling website.

I'd been wondering how much difference there was in my low-garage-capable HF antenna (Hustler resonators on a 36" mast) versus a longer antenna.  So late last week I put on the longer 54" mast, used a couple of springs in line, and put a long whip on the 15m resonator to tune it up on 20m.

The low-clearance setup was around 5.5 feet in total length, and poked above the roofline of my car by only 1.5-2 feet or so.   The tall antenna is about 9.5 feet tall, mounted in the same place on my car, so there is close to 6 feet of antenna above the roofline now.

I don't have calibrated field strength numbers, of course, but a couple of days of casual operation with the longer antenna (while not parking in the garage at work) has made it VERY clear how much the extra physical length helps:  With the short antenna, I needed to hear someone at S7 or S9 strength to have a reasonable chance at working him.  With the long antenna I can work people I hear only at S5 or S7; occasionally I can work someone who is only S3 on my receiver.  The short antenna gave me essentially no chance at someone I heard at S5 strength; with the long antenna, it's often no problem to work an S5 or even weaker station.

And I'm hearing that wonderful phrase often: "You've got a great signal for a mobile!" :-)

So you guys are right:  the extra antenna length makes a very significant difference, not a bit of doubt in my mind about that.   Now I've got to figure out what to do about that low parking garage clearance at work.  :-)
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WD5GWY
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« Reply #10 on: May 01, 2011, 09:32:49 AM »

It's true, size does matter!  Grin
(sorry, couldn't resist that one)
Getting more antenna above the body of your car helped a lot.
If you had the Little Tarheel II mountd on the roof of your car
you would seen better preformance from it compared to the
Hustler as you had it before adding length to it.
Although , not as good as a full sized screwdriver antenna, the
Little Tarheel II is better than the hamstick antennas. I use one
on an 18 wheeler mounted on the mirror mount (it's well bonded
to the frame Alan) and it works better than the 102" whip and
auto tuner setup I previously used. (behinded the cab )
I have worked stations that were as low as S4 or so in signal strength
(some were mobile too) so, height above the mass of your vehicle is
a big help. 
As for your parking garage situation, have you considered parking elsewhere?
Or do like some guys I have seen do, get a pickup mount the antenna (full sized
screwdriver) in the bed of the pickup attached to a remotely operated foldover
mount. Very expensive setup, probably more than the cost of your HF rig, but
you can have the full size antenna, plus be able to fold it over when pulling into
low places.
Being able to operate HF mobile is fun and to me that it the goal.

Good luck,
james

WD5GWY
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AC4RD
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« Reply #11 on: May 01, 2011, 12:00:38 PM »

James, thanks for the input.  I definitely looked at screwdrivers, and decided the Little Tarheel II or one similar is the biggest screwdriver I could reasonably mount on my tiny econobox car  (2006 Scion xB).   My conclusion is that the smaller screwdrivers weren't enough improvement over my hustler system to be worth the bother and expense.  But now I see a BIGGER antenna definitely IS worth some time and effort--I just need to figure out the best way to go from here.  :-)

I actually don't have another parking option, or not a good one.  Right now I park in a multilevel garage immediately adjacent to my workplace.  I get there early enough to have a great spot, and if it's raining, I don't get wet because the garage is connected to the building I work in.  There ARE non-garage options, but they're far away and full of crime.  So I'm lucky (in a way) to have a spot in this garage.  It's just a question of figuring out a way to keep a big antenna on my car and minimize the bother of getting in the low garage.  :-)  I looked at the motorized foldovers but they aren't usable on my toaster-sized (and shaped) car.  ;-)

I've actually been inactive on the radio for a couple of years, and I'm not finding much time to play on HF from home.  But I'm in the car an hour a day, and I'm having a world of fun on HF from the car--you're absolutely right about how much fun it can be!   I've got around 194 countries confirmed from my home station, but I noticed I only had 30 or so countries while mobile, when I started playing on HF from the car.  Now I'm up to 51 worked from the car and having a ball on my commutes.  :-) 

Thanks and 73!  --Ken AC4RD
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K0BG
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« Reply #12 on: May 01, 2011, 02:22:54 PM »

It isn't just the physical size. More correctly, it is the effective electrical length. You can have a short vertical antenna, with a large (properly mounted) cap hat that will be more efficient, and have wider bandwidth of a much longer one without any top loading. Do everything correctly, it is indeed possible to increase the radiation resistance by a factor of four. This requires minimizing base capacitive loading, and maximizing top capacitive loading. These requirements, almost always require drilling holes, and unfortunately, today's amateurs have an aversion against doing that.

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M6GOM
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« Reply #13 on: May 02, 2011, 05:45:51 AM »

These requirements, almost always require drilling holes, and unfortunately, today's amateurs have an aversion against doing that.


I used to think that magmounts and gutter mounts were perfectly good until the day I was forced to drill a hole due to common mode I couldn't get rid of.

Every antenna installed on any car I own in the future will be mounted by drilling holes.
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KP4MD
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« Reply #14 on: August 15, 2011, 11:44:59 AM »

In 2006, I moved to a mobile home park with covenants that restricted antennas visible from the street. My first antenna at this location has been a remotely tuned base loaded 6 foot vertical whip antenna (High Sierra Sidekick) mounted on the roof at the rear of the home. The aluminum siding serves as its counterpoise. Its observed performance is fair on 21 and 28 MHz, mediocre on 7 and 14 MHz with little reception except to the north, and overall quite poor on 3.5 MHz. I have posted a NEC modeling analysis of the Sidekick at http://www.qsl.net/kp4md/modeling.htm#sidekick - As it is tunable across the entire HF spectrum, it does allow operation on MARS frequencies. Screwdrivers are poor performer for their high cost, definitely best suited as a compromise for use on vehicles, as low profile antennas or other very limited space applications.
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