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Author Topic: Screwdrivers antennas vs. remote tuned whips.  (Read 754 times)
KP4RL
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« on: November 21, 2006, 09:44:23 AM »

Hi friends!

Which antenna you thing will perform better in the top of a building: a screwdriver with 8 feet whip and radials...or the 8 feet whip and radials but with a feed point remote tuner?

Thank you very much!

KP4RL
Hernan
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KA4KOE
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« Reply #1 on: November 21, 2006, 09:56:02 AM »

Screwdriver.

Goto www.k0bg.com
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N3OX
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« Reply #2 on: November 21, 2006, 10:01:03 AM »

I'd expect the screwdriver inductor to be rather higher Q than any of the inductors in even good autotuners, so I would think the screwdriver would end up more efficient.

Probably depends a little bit on the method of coil contact in the screwdriver, though I'd still expect it to beat the autotuner.

Now, a really good tuner with excellent components (like a nice old motor driven military unit) would probably come in as good or better than the screwdriver.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W5DXP
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« Reply #3 on: November 21, 2006, 11:02:42 AM »

In actual 75m mobile shootout contests, the screwdriver won by 12 dB, i.e. radiated almost 16 times as much power as the autotuner plus whip.
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73, Cecil, w5dxp.com
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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.
N6AJR
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« Reply #4 on: November 21, 2006, 12:20:31 PM »

I say the screwdriver, and the better the ground plane the better it will work.
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KB4QAA
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« Reply #5 on: November 21, 2006, 12:43:42 PM »

The screwdriver, because you are actually adjusting the length of the antenna to be resonant, vice having a non-resonant antenna and tricking the transmitter with a tuner.

bill
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AA4PB
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« Reply #6 on: November 21, 2006, 12:51:57 PM »

A good screwdriver will be better because the loading coil is made with larger wire and thus has less loss than most tuners. A tuner at the base of the whip will tune the antenna to resonance in a similar manner to the adjustable loading coil in a screwdriver. In either case, nothing is "tricking" the transmitter. The antenna is resonant when the reactance is zero.
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AA4PB
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« Reply #7 on: November 21, 2006, 12:57:39 PM »

By the way, no screwdriver adjusts the length of the antenna to achieve resonance. They adjust the tap on the loading coil to obtain the correct amount of inductive reactance to exactly cancel the capacitive reactance created by the physically short length. Some screwdriver whips do extend a little when tuning but that is a simple mechanical result of moving the tap up and down the coil. The length doesn't change enough to have any significant impact on resonance. In order for a screwdriver to adjust its length to achieve resonance on 80M it would have to extent to 67 feet.
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W5DXP
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« Reply #8 on: November 21, 2006, 04:39:41 PM »

> AA4PB wrote: By the way, no screwdriver adjusts the length of the antenna to achieve resonance. <

When someone makes such a statement, they are probably talking about the *electrical length* of the antenna, not the physical length. The screwdriver coil is adjusted until the reflected wave is in phase with the forward wave on the standing-wave antenna. That causes resonance and cannot be done unless the antenna is an *electrical* 1/4 wavelength long. For the reflected voltage to be 180 degrees out of phase with the forward voltage at the feedpoint, the voltage wave must shift by 180 degrees so the antenna must be *electrically* 90 degrees long.

To understand such a concept, consider that 27 degrees of a Z0=5000 ohm transmission line plus 11 degrees of a Z0=500 ohm transmission line is a 1/4 wavelength stub. Understanding how a stub that is physically 38 degrees long can be electrically 90 degrees long is the key to understanding loaded mobile antennas.
--
73, Cecil, w5dxp.com
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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.
AA4PB
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« Reply #9 on: November 21, 2006, 06:15:12 PM »

True. I also submit that a tuner located at the base of a short whip is tuning the antenna to resonance just as though it had a loading coil at the base. Other than the fact that a tuner probably has smaller, less efficient, coils than the typical loading coil and center loading is generally a bit more efficient, the result is the same. The antenna gets tuned to resonance. The tuner is NOT "tricking" the radio into working with an non-resonant antenna.

The only thing that affects the efficiency is the physical length of the antenna (the radiation resistance) and the losses (can I couple power into it efficiently).

Other things to consider in the screwdriver vs. tuner/whip systems is that if you use an auto tuner like an SGC or Icom AH-4, you can change bands instantly. With a screwdriver you will have to wait for the motor to adjust the tap on the loading coil. The whip is also lighter, has less wind resistance, and is less obtrusive. But a full sized screwdriver is more efficient, especially on the lower bands.
 
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N3OX
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« Reply #10 on: November 21, 2006, 09:09:42 PM »

Seems like a screwdriver is going to need another matching component to get up to 50 ohms... I guess some of them use ununs or some such... but if you just tune an 8 foot whip to resonance on 80m, it's going to come in something like 2+j0 ohms, right?  The method by which you match that to the line is going to have some impact on efficiency.

Still, an autotuner with powdered-iron toroidal inductors and a bunch of relay contacts in series with the inductance isn't a great choice.

One could build a remote tuner that would be closer to instantaneous and still do the job.   You'd need a big, airwound coil similar to or better than the screwdriver's coil and a remote switch to switch taps on it, (see http://www.n3ox.net/projects/lowbandvert/ for a possible switch arrangement),
but I think you could do a tapped-coil matching network this way on all bands (an L-L L network)

A problem would be that you'd need an awful lot of taps to cover all of a band.  I'd need five or so taps to cover all of 80/75m on my *forty* foot vertical this way, just based on the 2:1 SWR bandwidth at a single tap position on the vertical here (not more than 100kHz)

Still, if you liked particular parts of the bands you were working, it might not be so bad to trade off continuous adjustment for switching speed.

I personally wouldn't give up 12dB of efficiency for band switching speed, but if you build your own remote tuner you don't have to.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KP4RL
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« Reply #11 on: November 22, 2006, 05:38:01 AM »

Hi friends!

I appreciate very much your comments and point of views.  

I brought the question because I was thinking to change my antenna for a screwdriver.  

Righ now, the antenna Im using is a 9 feet CB whip + 4 feet hustler mobile mast + 9 feet linear coil at the base = total physical lenght of 13 feet and electrical 22).  About 5 radials of 10 to 22 feet, a 4:1 balun and the RT-11 LDG remote coupler at the feed point (BTW excellent coupler and customer service).  The thing match this "antenna" in any band from 160 to 6 meters with some "holes".  I live in the top of a three story building with some restrictions, but the antenna is thin and completely invisible from the ground.

The antenna works very well from 17 to 6 meters.  But sometimes I feel that 20 and down is deaf and dummy.  With your comments and ideas I did a new search in Google.  Now I understand that Im not losing power in the coax...but a lot of power in the tuner's tiny toroids is lost.  

My option was a screwdriver antenna based on the reviews and rates.  But a motorized 4 feet coiled PVC at a vertical multiband price...I should think it again.      

After all...now I think that my old retired rusty but trusty 44 feet zig zag doublet was a better solution.

KP4RL
Hernan
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AA4PB
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« Reply #12 on: November 22, 2006, 11:40:17 AM »

You can't really expect a whole lot out of ANY 6-foot antenna on 40M and below. That includes the tuner/whip or a screwdriver or any other antenna designed for mobile use. The typical mobile antenna on 75M radiates about 2% of the power you apply to it. The other 98% is disipated as heat in the various losses. They are used in mobile installations because there aren't any efficient options. For a home installation you would be FAR better off with your 44-foot wire. Good antenna efficiency depends on high radiation resistance and that requires a physically large antenna for the lower frequency bands.

To radiate the maximum power you need high radiation resistance and low loss. If you double your 6-foot whip to 12-feet, you double the radiation resistance. Since the loss in the tuner and grounding system remain about the same, you double your efficiency. Make that whip 44-feet long and you are starting to get some pretty good efficiency.
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