From: Patrick Jankowiak (eccm@swbell.net)
Date: Wed Oct 16 2002 - 22:12:04 PDT
Antenna tie-downs and SWR issues:
I have installed one of those military low band VHF antennas on my M35.
This is the vehicular type used with the PRC25, PRC77, RT246, and RT524.
It's the more modern type (AS-3684/VRC) which does not require manual or electronic tuning, and just has a BNC connector on the botom of the base mount, and the mast is a two-piece type about 7 or 8'
tall, where the bottom section has a coaxial conection inside its lower end, and the top screws into the top end with a single conductor. Both piecs are 'fat', meaning about 3/4" diameter. the top
section tapers to about 1/4". Anyway, bad description, common antenna.
I welded a sugar scoop to the backside of the front bumper, and mounted it there. After doing some checking, I noticed that the SWR is pretty good from about 30-90 MHz. It's 3:1 or less over the whole
range, with a couple sweet spots of 1.5 or 2 to 1. Considering that 3:1 means that 75% of the power is going out the antenna, it's fine.
I decided that this antenna needed to be tied down a bit, and I happened to have the kit. It's a metal clip, about 3" long and 1.5" wide, which fits onto the antenna near the top, and has an attached
lanyard which you tie to whatever. I tied it to the windshield bracket release handle. Tied down, the swr really went sour, it was 3:1 at very best, and often 6:1 to 25:1!
At first I thought it was from the tip being tied so close (7-8") to the metal of the cab windshield, but it was not the case. The tip of an antenna is usually the most sensitive part when it comes to
proximity to other objects.
The metal clip of the tie down kit is what is causing the bad SWR. Apparently, when it is intimately in contact with the antenna shaft, it adds alot of capacitance to the antenna, which is already a
complex tuned system (the base has a special broadband matching network inside, that's why there's no knob or large plug for the tuning control), and the metal clip upsets this delicate balance. I
proved this by just resting the antenna in the clip's edge so that the antenna was in the 'tied-down' position, not fully enclosing the shaft in the hook of the clip, and the SWR was not as bad. I
removed the clip, and I bent the antenna by hand into the position where it had been tied, and the SWR was quite acceptable.
Therefore, I suggest a non-metallic loop of material instead of the metal clip, for tying down antennas. A black or camo-colored nylon dog collar comes to mind, or a robust plastic snap-hook of some
kind. Whatever is used must completely encircle the antenna, or it may jump out of an open ended hook during spirited maneuvers. Any ideas?
An MFJ antenna analyzer was used for the SWR tests.
---On a humorous note, Iraq TV news just reported that Saddam Hussein recieved 100% of the vote in the presidential referrendum. 100% of all eligible voters turned out, and no other candidate was on the list. Democracy in action!
This archive was generated by hypermail 2.1.4 : Wed Apr 23 2003 - 13:24:09 PDT