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Author Topic: HF Mobile Antenna Question  (Read 9090 times)
W5LZ
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Posts: 477




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« Reply #15 on: February 22, 2011, 07:42:32 AM »

Just an observation about using a tuner (doesn't matter which one) and a 102" whip.  At frequencies around 20 meters and above, it's 'usable' to fairly good.  Below 20 meters it only gets worse, and on 40 and 80 meters, don't waste your time.  No 'numbers' or other things to support that, just what I've heard from people I know who have tried it.
Paul
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WX7G
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Posts: 6194




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« Reply #16 on: February 22, 2011, 09:42:16 AM »

There is a way to make the whip work much better at 40 and 80 meters and that is to install a good loading coil at the base. Then the tuner does impedance matching but does not have the burden of resonating the whip. With the arrangement performance similar to a screwdriver can be obtained.
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WB6BYU
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Posts: 13454




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« Reply #17 on: February 22, 2011, 10:28:44 AM »

Quote from: WA6BJH
...It can't come from ground losses because we can't change the ground losses...


Perhaps you can't change the ground loss, as measured in ohms.  But you CAN change the
amount of power dissipated in the ground loss, and therefore the dB impact it has on your
signal.

Say your ground loss is 10 ohms.  If your antenna has a radiation resistance of 0.1 ohm and
ignoring all other losses, you'd have an efficiency of about 1%.  If you redesign your antenna
to have a radiation resistance of 1 ohm, then you can raise your efficiency to 9%.  You've
improved your signal 8dB without changing the ground loss.

One way to increase the radiation resistance is to move the loading coil up higher on the
whip.  A base-loaded antenna (which would be the case using a tuner at the base of a whip)
is about the worst case in this regard.  Putting the loading coil higher on the antenna and
using a top hat can make a big difference.
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W5DXP
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Posts: 3639


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« Reply #18 on: February 22, 2011, 11:38:19 AM »

There is a way to make the whip work much better at 40 and 80 meters and that is to install a good loading coil at the base. Then the tuner does impedance matching but does not have the burden of resonating the whip. With the arrangement performance similar to a screwdriver can be obtained.

Yes, K7JEB's base-loaded Texas Bugcatcher coil with CB whip configuration was less than one S-unit down from the average screwdriver or center-loaded bugcatcher without top hats in the CA shootouts.

One gains ~3 dB going from base-loaded to center-loaded because the center-loaded antenna has approximately double the radiation resistance of the base-loaded antenna on 75m (0.8 ohms Vs 0.35 ohms, according to The ARRL Antenna Book).

One gains ~2 dB by installing a good top hat.

Jim, K7JEB, and I drove down I10 from Phoenix, AZ to LA for the Visalia, CA shootout. We were talking to each other on 3.95 MHz, a popular mobile frequency in that area. Signal reports from other hams indicated that my top-loaded antenna was one S-unit better than Jim's base-loaded antenna.
--
73, Cecil, w5dxp.com
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N3OX
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« Reply #19 on: February 22, 2011, 12:54:54 PM »

If I have a mobile HF antenna on my car, where are the losses?  The DC resistance loss is obvious; it's the DC resistance of the antenna rod and the coil.  This is not going to change much from antenna to antenna.  

It's not the DC loss that matters.  It's the RF loss of the whole assembly.

You can get a surprising amount of loss variation between objects made of the same amount of the same wire.  If you put too much capacitance in parallel with the coil, you get a parallel LC circuit that transforms the very low resistance of the wire up, with the loss coming from huge circulating currents flowing through low resistance.

You can take any parallel LC circuit ( a real one, with resistance) and find arbitrarily high values of inductive reactance on the low side of the resonance curve.  But the closer you get to self-resonance, the higher the resistance gets and the worse the Q gets.  This will be more of a problem in antennas that were designed by a "wind some wire on a stick" process.. if you have a solid prediction for how many turns you should need of "pure inductor" you can largely avoid this problem.  If you know you need 2000 or 3000 ohms of inductive reactance and you're getting that with a fraction of the turns you'd expect, it would be a big red flag that you were climbing up the inductive side of a parallel resonance peak instead of building a decent, high Q 3000 ohm inductive reactance.

I don't know how much this affects the autotuner v. screwdriver debate, but it's not good to assume that the loss resistance is low based on the estimated low frequency resistance or even the RF resistance based on some skin depth corrections.  Capacitance shunting an inductor could transform a couple ohms of wire loss up to a considerably larger value.


« Last Edit: February 22, 2011, 01:05:57 PM by N3OX » Logged

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

Monkey/silicon cyborg, beeping at rocks since 1995.
WA6BJH
Member

Posts: 58




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« Reply #20 on: February 23, 2011, 03:47:59 AM »

When you find the report, let me know.
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