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Author Topic: mobile HF on 75 meters  (Read 481 times)
AB9DF
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Posts: 81




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« on: January 20, 2005, 10:40:07 AM »

I have started doing more and more HF work while driving on Business trips.  I have a TS-450 and a hamstick on 20 meters during the day and I can easily work the world on 100 watts.  At night, I switch to a 75 meter ham stick and then all my problems begin.  Even on 40 meters, I just cant seem to get a good setup.  Is it inhearently more difficult to operate the lower bands on a mobile setup?  In my situation, and I admit this is weird, my reception seems to be good and then fades away.  I then only have to touch the connector of the coax and then suddenly the sound comes back.  It is sort of like I am releasing static electricity or something.  Anyone ever have this happen? It is really annoying.  My antenna is a tri-mag mound.  Not sure what the problem is but it might I suppose be the ground but it is grounded.  Any insight would be helpful   73
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K0BG
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Posts: 9863


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« Reply #1 on: January 20, 2005, 11:28:45 AM »

First of all, I don't like nor recommend mag mounts. They can and do come off if you have a crash, no matter how many magnets they have.

One of the most misunderstood aspect of mobile operation is the need to maximize what little groundplane there is. As the frequency goes down, the more important this is.

On 20 meters, an effective radiated power level of 10 to 20 watts (about all you have with your current setup), is enough to work the world if the band is open. On 75 meters with the same setup, your effective radiated power is in the order of 2 to 5 watts.

Rolling static is no more of a problem on 75 than it is on any other band, it is just that it effects the receive signal to a larger degree. It is also indicative that your antenna has a poor DC and RF ground. And most likely, you don't have proper bonding.

This is a subject too complex to discuss here. If you want some insight, visit my web site.

Alan, KØBG
www.k0bg.com
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W7DJM
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Posts: 1




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« Reply #2 on: January 20, 2005, 07:28:47 PM »

You either have an electrical intermittent connection, or the mag mount is seriously affecting antenna conditions.

You REALLY need to get rid of the mag mount.

The only real problem I know of with lower bands mobile, is that the antenna, ANY antenna becomes less efficient, and therefore you need the best you can afford to make the thing effective.  Hamsticks would not be my first choice, but they will get you on the air.

Also, lately, propagation has not been "the best."   A few years ago, I worked Hawaii, twice, on 75 mobile.
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K7FE
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Posts: 5




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« Reply #3 on: January 22, 2005, 09:58:19 PM »

DF,
You need a decent antenna for 40 & 75.  A Ham Stick is such a poor performer on those bands that I'm sure it must not be fun to operate.  I suggest a "good" Screwdriver or Bug Catcher.  Bigger really is Better.  My Bio on QRZ.COM describes my homebrew Bug Catcher HF Mobil antenna that can be built for less than $50.
73,
Terry, K7FE
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KF6RDN
Member

Posts: 39




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« Reply #4 on: January 26, 2005, 10:32:43 PM »

I agree with the others, if you can, get rid of the mag mount. they're fine on vhf/uhf, it's capactive coupled to your car body, at those frequencies it's a non issue.

HF, however, 'specially lower 40-80 mtr it IS an issue. You could be having dirt/moisture hell birdcrap effecting the coupling.

I have a sedan type car, mounted on the trunk, trunk is strapped to car body, used a 3/8" "acorn" mount from rat shack. It's admittedly borderline mechanically as in if I whack something really good, I stand a chance of torqing a good bend in my trunk, though it did ok when I drove into the garage, forgetting to take it off, it broke the quick disconnects and the hamstick, didnt hurt the trunk.

Anyway if I take the antenna off, it's not too bad looking, the hamsticks do ok, they are long, but low profile if that's an issue.
They will compare to most of the other fixed bandwidth antenna, hustler etc.
Yes a screwdriver would work better, 'specially on 75 with 75kh or so bandwidth of the sticks..  If you don't mind the "ugly" factor, the convienance of "surfing the bands" may make it worth it..
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WA4JM
Member

Posts: 19




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« Reply #5 on: February 01, 2005, 11:55:54 AM »

Don't let anyone fool you, a Hamstick on 75 meters is a dummy load.  The small gauge of wire used to wind the antenna eats up most of your power.  This is an antenna that "When you hear 'em, you can't work 'em."

Also I agree with the others, ditch the mag mount.

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