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Author Topic: Wire from screwdriver antenna tip to car bumper/fender?  (Read 6899 times)
HS0ZIB
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Posts: 424




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« on: August 05, 2014, 11:49:13 PM »

I have a screwdriver antenna with whip mounted on a horizontal metal bar that is installed behind the cab on my Toyota Hilux, bonded to the truck bed etc.

This set-up provides me with good greyline/late afternoon coverage and the whip antenna is not too high to obstruct driving.

But QSOs during daylight hours have not yet been achieved, (10mb seems 'dead' now...)

On the basis that more antenna 'metal' is better, has anyone tried a set-up where a thin/strong metal wire is attached to the tip of the whip and then run diagonally down to an insulated fixture at the front (or rear) of the car?

Since I'm using a screwdriver antenna, the increased whip+wire length should still be tunable to resonance.

I am trying to find a solution which improves my HF mobile signal, whilst still allowing QSOs whilst driving the car, (ie whip antenna length not excessive).

Especially, I want to be able to have daytime QSOs whilst mobile.
« Last Edit: August 05, 2014, 11:52:27 PM by HS0ZIB » Logged
G8YMW
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Posts: 237




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« Reply #1 on: August 06, 2014, 02:44:31 AM »

I am surprised you cannot get any daytime QSOs?
Which bands are you trying on?
I would have thought there would have been some bands open eg 14MHz, 18 MHz and 21 MHz?
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73 details Tony
Sent by WW2 Royal Navy signal lamp
HS0ZIB
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Posts: 424




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« Reply #2 on: August 06, 2014, 03:03:42 AM »

Quote
I am surprised you cannot get any daytime QSOs?
Which bands are you trying on?
I would have thought there would have been some bands open eg 14MHz, 18 MHz and 21 MHz?

Part of the problem is that my location (south Thailand) generally precludes QSOs in the < 500Km range because there are no other active hams - I need to have a daytime QSO range of 500 - 2,000Km.

I listened on those bands around local midday today.  I could hear 1 Chinese ham (in Chinese language), but none from Thailand or neighbouring countries (and have never heard these in the daytime).

The nearest country where I have regular QSOs is across the Indian Ocean to India ==> 2,000Km.

Evening is a different affair, with QSOs from Brazil via greyline, and daily to Europe, Middle East, Australia etc.  Evening on 40m I hear stations in Malaysia and Indonesia.

So it isn't really that my mobile set-up is 'poor'.  My need is rather to absolutely optimize that set-up to enable daytime 1,000km QSOs.
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K0BG
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« Reply #3 on: August 06, 2014, 01:20:31 PM »

To directly answer your question....

Length matters because radiation resistance is a square function of the electrical length, and how the current flows over that length. Thus a 9 foot antenna has twice the radiation resistance than a 6 foot one. However, when you bend over the whip towards the vehicle, you induce a lot of stray capacitance which degrades the performance. more than the extra length helps.

Your best bet is a properly installed cap hat. Done correctly—mounted at the very top of the antenna—the radiation resistance can be increased by up to four times, particularly on the lower bands. The occurs because the current node in raised higher in the antenna.

You might want to read this: http://www.k0bg.com/caphats.html
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WB6BYU
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Posts: 13335




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« Reply #4 on: August 06, 2014, 02:59:22 PM »

One problem with running a wire from the top of the antenna down to the bumper is that the
current in the wire is out of phase with that in the antenna itself above the loading coil.
That makes it less efficient.

Studies at VLF show that capacity hats sloping downwards improve signal as long as the
ends are at least half the height of the antenna itself, and that this is is relatively constant
independent of hat width.  So making a wire extension may help somewhat as long as the far
end is at least as high as the midpoint of the antenna itself.  (I don't remember whether the
original tests looked at antennas with inductive loading in the middle of the radiator.)  But
since the base is elevated, you'll have to use only enough wire to reach down to the middle
of the antenna, with insulating rope from there to whatever anchor point you have available.
That will limit the amount of additional loading that you can use.

You also want to keep the total antenna + wire length to less than 1/4 wavelength or you can't
match it with the series coil.


I did try it once with a 9' bumper-mounted whip and an extension wire to tune it close to 15m.
It seemed to work, but I didn't run a good comparison between that and the whip by itself to
know if it actually made any improvement.
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N5UD
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Posts: 803




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« Reply #5 on: August 08, 2014, 07:40:05 AM »

Simon, in regards to caphats. I built one similar to Alan's. It makes a difference for sure on 40/80M. Maybe even 30M. I have mine rigged with a quick disconnect.

In wooded east Texas I can't go down the usual roads with the loops caphat. OK on a major highway. The whip is OK all the time just slapping those trees. The caphat makes my high frequency limit be 17M. Have to go to whip only for 15M +.

Thanks Alan for the caphat design idea.

73 Tony N5UD /M
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KB1GMX
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Posts: 780




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« Reply #6 on: August 21, 2014, 12:39:35 PM »

There was an antenna constructed for 80m in the ARRL Mobile Manual (1968 edition) years
back that was a short vertical section with a long horizontal section and a coil load plus hat
at the end.  It was called the Texas Longhorn.  The author mounted it on a 1964 station wagon
as a solution to the problem of height vs length for low HF use.  It is effectively a coil loaded
short inverted L.  It was also in QST august 1967.

Effectively adding a wire to your screwdriver does that save for sloping down is less helpful.
If you could make a mount at the bumper of the truck and hold the end up about even with
the tip that would help greatly.   Since it could run to the rear its less likely to catch branches.

Allison
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