Sunday, June 23, 2013

Antenna Topics: Using your mobile station as your base antenna. Post #204

An article by retired ARRLTechnical Editor Joel Hallas (W1ZR) in the July 2013 issue of "QST" offered me another idea of operating in "stealth mode."  Hallas outlines a simple way to use your mobile antenna as a base antenna in areas where restrictive CC&Rs and HOA's prevail.  With a few simple components, you can use your vehicle-mounted antenna as your main antenna when everything else fails.

Thanks to the use of two Bias Ts (devices that allow dc to share a coaxial cable without interferring with the RF), " you can easily supply power from your home shack to the vehicle's antenna system."  Hallas continues by saying that "the bias T at the end of the coax removes the dc.  The dc can either be used to power an automatic antenna tuner, or provide power to run a motorized antenna."  Hallas says "the usual  'monoband' mobile whip, either those with lumped loading inductance or helically loaded types, can be used on lower frequency bands than their self resonant frequency if used with a wide band tuner."

All you would have to do is run coax with the two Bias Ts from your rig's power supply to the automatic tuner or motorized antenna controller in your vehicle.  Now, you're in business.  Be sure to detach the coax before you drive away.  Hallas recommends leaving a reminder in your car and on your home transceiver to coil up the coax after you close out your operating day.

Hallas also describes a modification of this arrangement using the popular "screwdriver" antennas supplied by Tarheel and other manufacturers.  All told, Hallas's article gives several antenna ideas for those caught in the web of HOAs and CC&Rs.

Using Hallas's  general idea, I tried out a slightly different version for my situation.

I had several of those old "Hamstick" helical mobile antennas in storage just waiting for another antenna mission.  I also had an old MFJ mag mount that could be pressed into service.

At 11:30 a.m. local time today (Sunday, 23 June 2013), I drove my van into the back yard where it couldn;t be seen from the street.  I placed a 40 meter Hamstick into the mag mount and positioned the mount in the center of the van's roof.    I attached three 33-foot (10.06 meters) radials to the base of the mag mount with large alligator clips.  Hopefully, these radials would improve antenna performance.

I connected 50 feet (15.24 meters) of RG-8X to the Drake MN-4 antenna match box and ran the cable to the mag-mounted Hamstick.  With the use of the Drake MN-4, I was able to keep the SWR below 1.3 to 1 across the entire 40 meter amateur band.  Running approximately 20 watts output from the venerable Swan 100-MX, I made several local contacts.

I repeated the procedure with a 20 meter Hamstick  using three radials cut for that band and found no problem making contacts with the mag-mounted Hamstick.  I tried the same efffort with 15 and 10 meters.  Those bands were quite noisy, most likely a result of the recent M-Class flare from the sun.

Although performance of the emergency home antenna didn't match what I usually found with the 40 meter loop or the 40 meter inverted vee, I was satisfied that the "home-based" mobile worked well enough for emergency purposes.

If you follow Hallas's instructions, or even my humble efforts, you should be able to continue making contacts despite the restrictions you face.  Hallas concludes his excellent article by saying "In any of these cases, you have the ability to operate from the comfort of your home station with an antenna that works reasonably well."  Remember the old saying:  "Out of sight, out of mind."

REFERENCES:

Hallas, Joel, W1ZR.  "Use Your Mobile Station as a Base Antenna Platform."  "QST", July 2013, pp. 39 to 40.
Ford, S., WB8IMY.  "Short Takes--MFJ Model 4118 Bias Tee Power Injector."  "QST". May 2008, p.54.
Salas, P., AD5X.  "Build a Legal Limit Bias T that Covers 1.8 to 230 MHz."  "QST", Jan. 2013, pp.45-48.

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Aloha es 73 de Russ, KH6JRM

BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.