<< The High Sierra and other "screwdriver" antennas will be too tall for
mobile operations from a tall SUV like the Durango (or indeed from any
vehicle). However, it can be mounted to the frame and come from underneath
the bumper. I think there's
a picture of such an installation in this or last month's QST; I think they
did a review of this [type of] antenna too.
Insofar as the antennas mentioned, I've used all three successfully
although I was really portable rather than mobile. I would drive up to
Laramie, Wyo from Denver and then install the antenna on the roof of my Ford
E350 van using a four-magnet base. The Valor is a good choice money wise
(compared to the Hustler). I would go with that or the Hustler (I think the
Hustler can handle more power) if you want multi-band capability without
having to stop. For single band op or where stopping to change bands is not
an issue, go with the Hamstick. 73, Bob AA0CY
Here is a great mobile and base Z matching and broadbanding trick:
In regard to your comments above I do not believe that the Hustler will
handle more power on 75. I took one apart and it's wound with Litz wire
which has more resistance above 2 MHz than the same size solid wire. Most
coils on 75M will give about a 1.3:1 SWR for a Z of about 35 ohms. The
Hustler gives 1:1 on 75 which indicates to me it has a higher Rloss due to
the Litz wire. Litz wire is what is wound on the Toroids in BC radios and it
has less loss than the same size solid wire there. Try different coils and
the one that gives you the highest SWR at resonance has the least loss.
There is a sneaky trick of adding a BC 3 gang variable in a box and in
series with the center coax lead only at the feedpoint. Resonate the antenna
at 3.785 MHz with a plate shorted at full capacity (straight through). Open
capacitor and the resonant freq. will jump to about 3.8. The same 20 kHz
bandwidth occurs. Load the whip at say 3.825 and tune the BC variable for
min SWR. It will be less than 1.3:1. Load it at 3.85 and it will be about
1:1 there. At 3.9 it will be about 1.5:1. So you have about 100 kHz of
tunable bandwidth less than 1.5:1 just by adjusting the BC variable through
about 160 degrees of rotation. No outside adjustments of the whip coil.
What is happening is as you operate higher in F the whip is inductive
reactive which is tuned out by the variable Xc? But since the antenna is
approaching a half wave in length and a higher Rr value at a much faster rate
than with full size antennas due to the loading, the Rr value rises quickly.
I make use of this concept. Current through Rr creates radiation. Current
through the Rloss creates heat but this is constant except for the reduced
feedpoint current of the higher load when the Rr increases.
Formulas show that the Rr for a 75M whip is 1 ohm. So at 35 ohms or
1.3:1 SWR, you have 1 ohm Rr and 34 ohms Rloss or about 3% efficiency. But
with the Hustler the Rloss is greater and with a 1:1 load, the SWR losses are
lower but no big deal on 75M with 100 W--1 ohm Rr and 49 ohms Rloss, 1:1 SWR
and 2% efficiency.
At 3.9 the load is about 75 ohms (1.5:1 SWR) and the Rloss is still 34
ohms so the Rr must be 41 ohms. That being the case the efficiency should be
over 50%.
Try it and see what you think. I may resonate it at say 3.6 and operate it
at 3.8. The R part of the Z will be much higher. I am shooting for about
300 ohms Rload with the right amount of series Xc. Tune out the reactance
and use a low loss L network to lower it to 50. That should give even higher
efficiency. Driven elements don't have to be resonant if you known how to
match and make the new loads beneficial. Mobile is where the Rr is so low
and where this is really beneficial to use the series Xc trick. I made use
of the fast Rr increase. I've done this on low Rr yagi's. If it weren't for
high loss mobile coils on 160, 75 & 40, no match would even be close in order
to get any RF out of the xmitter. This works great on 160 also. If the new
SWR with the series Xc in the line is higher, the coil resistance is too
high. But sadly most goes into heat. Feel the coil after a long xmission.
For Screw Driver Antennas one adjusts the resonant length of the antenna
and there is a series Xc value that will give 50 ohms for that band in the
middle--record the Xc setting for each band. The antenna length is then
changed with a switch at the operating position instead of varying the
variable Xc at the base of the antenna which may not be close or even
outside.
Put a coax switch in the coax so that you can switch in the MFJ analyzer.
It will tell you right where the antenna is resonant when you are changing
the lengths and
it will save you a lot of time. I am going to write this up and send it to
one of the mags. I get many comments back from those who I showed this
technique to. High Rr feedpoints are always more efficient and is one reason
I like to end feed half waves. More on that later.
Tip low frequency whips back at 45-80 degrees. They work better that
way locally and a ways out. If the top whip gets too high, extend it back
horizontally--it works even better. The series Xc will work with any length
of whip and length extension. This is the ultimate of simplicity and maximum
performance K7GCO style.
I've had great signals mobile using the Series Xc with the TJ
Screwdriver Antenna that was made in Or.by Tom Wilson. I have 1:1 over all
bands as a result of the right series Xc value. It's now made by Don Nott,
Nott Ltd. 4001 La Plato Hwy, Farmington, New Mexico 87401, 505 327 5646. I
was planning to mount it on top of my house with radials. Other whips
mounted on my house with radials worked great but I had no way to tune them.
It would be great for metal mobile homes or on top of apartments. The car is
not equivalent to a 1/4 wave on 75.
K7GCO Ken Glanzer
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