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Author Topic: SWR help on FORD Winstar  (Read 1000 times)

Posts: 43

« on: May 14, 2004, 06:53:46 PM »

I am fairly new to ham radios so I yet again have another question.  I have a 2m mag mounted antenna and a Icom V8000.  The antenna is a 5/8 wave.  I hooked it up to the Diamond SWR 40c, put it on low power and the SWR read 4.  I changed the location on the car kept retuning the antenna.  Than for some reason I decided to set the transiever and the SWR meter to high power and it read a solid 3.  Is there any way to improve the SWR.  Or am I just doomed for the time being until I can get the antenna you drill in?  Thanks for help....!

Posts: 10248


« Reply #1 on: May 15, 2004, 09:16:51 AM »

I must say I don't like mag mounts, but it isn't because they have high VSWR. You can not rely on ANY internal SWR bridge to give you a true indication. An external meter like a Bird will get you closer.

The best was is to beg, borrow, or steal an MFJ 259B or 269 and check the antenna with it. Then you'll know the actual input impedance and what the resonant point is.

Most out-of-the-box Larsen antennas will be very close using the cutting chart included with the antenna, if there is one. Some of their antennas don't require any cutting.

Alan, KØBG


Posts: 41

« Reply #2 on: May 15, 2004, 01:37:32 PM »

Why can you not rely on internal SWR meters? small physical size/low accuracy? Physical location in the circuit? Other?

On the Winstar is the SWR about equal a both end of the band? Is it possible there's a bad connector/connection at one end of the cable. I'm trying to learn!

Also, the weaknesses of mag mounts noted here and elsewhere, they would seem to have their place for temporary setups, in motion use w/ *short* whips (say 1/4 wave 2m/1.25m/70cm) and stationary even with 5/8'ths and colinears, no? (I even have used a base loaded perhaps 30" (~0.7m) whip -- long ago -- for (cough) 11 (cough-cough) meters, so a 19" whip w/o a loading coil seems as though it should be tame, and with 10-40W, or more, to boot a lot better than a rubber duck on a 5W or less HT, though (granted) no match for a bigger radiator solidly grounded at its base.

Contrary arguments? brickbats? other observations?

Also, I have seen references to 1/4 and 5/8 whips requiring STEEL surfaces for the ground plane, vs. aluminum (e.g. tops of campers/RV's, heavy truck cabs, etc.) Is it indeed true the ground plane needs to be ferromagnetic as well as electricly conductive? This seems to fly in the face of earth ground planes for ground mounted 1/4 wave radiators (ham, AM commercial, etc.) and for the aluminum or copper wire/rod radials on elevated verticals.

Posts: 10248


« Reply #3 on: May 15, 2004, 06:58:25 PM »

Internal SWR bridges are notorious for their inaccuracies. In some case, like the one in an Icom IC706, the SWR, power foldback, and over temp circuitry is one and the same. As the heatsink gets warm, the SWR indication goes up even if it is connected to a dummy load.

The surface below the antenna can be just about any conductive surface. In the latest issue of QEX, there is a good article covering the tests done with a variety of conducting surfaces including conducting paint and particalized plastics. There is hope!

Alan, KØBG


Posts: 2008

« Reply #4 on: May 16, 2004, 07:11:46 PM »

Forget the mag mount!

Remove the overhead console (remove one screw in the middle of the garage door opener compartment and pull the console straight down) and mount a 3/4 inch Motorola NMO mount theough the roof at the rear of the headliner opening. Install a Larsen 2/70 NMO mount antenna and run the coax through the headlined and down the piller. Reinstall the comsole. Takes about 20 minutes total.

Last dual band mount you will ever need.

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