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Author Topic: 75m/160m mobile Dxing  (Read 1166 times)

Posts: 12

« on: December 19, 2008, 06:08:14 AM »

I was wondering if anybody had any experience with 75m mobile dxing. I took a trip to Michigan from Nevada around the middle of November. Just about every night and morning I worked some kind of dx on 75m while mobile. I am running a Yaesu FT-857 @ 100w and an MFJ-1668 manual screwdriver antenna. I have (well had, since a tree in MI was a little too low) a 3’ diameter caphat made from 1” soft drawn copper tubing beat flat and drilled in the center. I put the 2 lengths together to form an X and mounted them at the top of a 102” fiberglass whip, tying the ends together with #14 copper wire, and running three #10 copper wires from the base of the whip to the caphat.

Anyway, I worked around 20 countries (around 100 dx contacts) on 75m while mobile and barefoot. I started out my trip by working ZL about an hour before sunrise from SLC, UT -- for some reason couldn't get VK, but could hear them just as good. I then worked a couple stations in the UK from eastern Wyoming late that evening. Also, I did all my contacts on SSB. While in MI I worked Europe every day I was there (6 days) catching eastern Europe on their sunrise and following the sun from country to country.

What I was wondering, was this unusual conditions or was this just because of an efficient mobile antenna? I could also work the west coast on 75m with no problems every night that I was in MI.
Oh, I did also work 1 station in Europe (Austria) on 160m from the mobile, which it was ½ hour after his sunrise!. For 160m I have a coil mounted to my bumper, running horizontally with a wire going to the base of my screwdriver.

I have searched high and low on the internet trying to find info on 75m and 160m mobile dxing and have came up empty. I now know that it possible to work Dx from the mobile on 75m and 160m.
I would like to try another antenna that will handle more power and hook up an amp, but I also don’t want to screw up and use an antenna that is not as efficient as the one that I using now. I was told that I was only down 20dB from the big stations as received in the UK, and they were running legal limit. So I figure that I am running around 12%-15% efficiency with this antenna on 75m, and probably around 2% on 160m.
Are there any of the motorized screwdrivers that  work this good with a decent caphat, or should I keep this antenna and continue running barefoot? My truck has an alternator with ample amperage, so that is not an issue if I were to hook up an amp. I have been looking at the HI-Q, but want to know a little bit more about them before I commit to a pricey antenna.

John, AD7VH

Posts: 10248


« Reply #1 on: December 19, 2008, 06:29:34 AM »

If you do EVERYTHING right, you might get to 8% efficiency on 75 meters. The overall effective length has to approach 14 or so feet, and you have to have as much mass under the antenna as possible. That means body mounted, not bumper mounted!

However, on the other end of the scale, even an above average 75 meter antenna with 4% efficiency will garner DX is the band is open. Since we're in the low part of the sun spot cycle, what you're seeing is typical. That is, if you try, as a lot of folks don't think DX on 75 is possible from a mobile. It is, obviously.

Lastly, the average 75 meter mobile antenna is about 1% efficient, and perhaps lower depending on what it is and how it is mounted. They don't do to well in a pileup, hence the reasoning about mobile DX.

I don't cover DX per se on my web site, but there is information there you might be interested in; amplifiers, wiring, etc.

Alan, KØBG


Posts: 10248


« Reply #2 on: December 19, 2008, 06:46:26 AM »

Let me add a few things now that I have reread your post.

Tom, W8JI, does a lot of 160 meter mobile. If I remember correctly, he's getting close to 5% on 160, but then again, most amateurs won't do what it takes to obtain this level of efficiency. It's a little easier on 75 meters as long as you're willing to drill holes, etc. You can visit his web site for more information.

Obtaining mobile antenna efficiencies of  12% to 15% on 75 meters is virtually impossible, and would require an effective length of well over 20 feet. You can do that, but you aren't going to drive around with such a beast.

HiQ does have one model which approaches 12%. Because of the wind loading, overall length, its 6.5 inch diameter, etc., you can't drive around with it. The cost is just over $2,000.

In another year, perhaps two, DX opening on 75 and 160 will all but dry up except for the dead of night, and it will be rare then. Be thinking about moving up the bands in the next few months.

Alan, KØBG


Posts: 2086

« Reply #3 on: December 19, 2008, 06:57:27 AM »

I don't have the most efficient system, and on 160 I use a Little Tarheel II that is not rated for 160(works when parked with 12' telescoping whip). I have 23 on 160 and 74 on 80 meters and 51 on 75 meters. While on vacation on South Padre Island, TX last year I worked WAS in about 3 days off and on while enjoying the beach. The screwdriver can handle 200 whats from my little home brew amp and I also use the FT-857D. Some day I'll get a bigger car, bigger screwdriver and bigger amp. For now I'm happy. Have fun!

73 de Lindy

Posts: 14491

« Reply #4 on: December 19, 2008, 03:28:44 PM »

Which explains why people work loads of dx running 5W to an efficient antenna.

Bob  AA4PB
Garrisonville, VA

Posts: 12

« Reply #5 on: December 24, 2008, 01:34:30 AM »

Thanks to everybody that replied. As always, I knew that the first person to reply would be Alan, K0BG. I always respect what you have to say, Alan, since you have never steered me wrong. It was because of your advice in the past that I set this antenna up the way that I did -- other than mounting it to the bumper.

I did model my pick-up and antenna using EZNEC. The results I came up with did surprise me. I thought that it would be a little better, but I came up with a gain of -13dBi. This means that you were right on the money, Alan, since this would mean it would be about 5% efficient. I did try another scenario with the antenna mounted in the box and the efficiency was around 9.5%. I definitely trust you more than ever, Alan, since what you said went right along with fairly precise models.

I am also very familiar with Tom's, W8JI work, and try to read anything and everything that he has published. His work is, to me, some of the most informative, but yet is simple and to the point. He is one of the only people that I know that has published anything about lowband mobile DXing.

I never really tried to do any lowband DXing where I live, as I am surrounded by very tall mountains, which makes DXing to any point other than over the north pole next to impossible -- this is the only direction that I have a clear shot.

Now that I know that it is very possible, I am going to have to try it from a couple of mountain tops that I access to.

Oh, Lindy, these countries that you worked were you by the ocean? As this makes one heck of difference.
The one advantage that I had when I when I was back in Michigan was that I was parked in an open field that has very high soil conductivity. This was my childhood home, so I am very familiar with it; a lot of SWLing and I also got my general ticket (ex-KC8NTP) while I was still living there.

73 and thx for the help, John, AD7VH
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