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Author Topic: 160m mobile/lowband mobile  (Read 858 times)
KC8NTP
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Posts: 16




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« on: September 05, 2007, 07:14:48 AM »

I was wondering what I could expect while operating 160m mobile. I have searched high and low online and cannot come up with very much information concerning 160m mobile operation.

I have an antenna system that I have devised for 160m, but I want to know if it works as well as it should. I am using an MFJ-1668 screwdriver, with another loading coil on top of that, a caphat with 18in spokes, and a 102in whip on top of that. I also have made an adjustable shunt: it is a 10 position rotary switch and 7 taps into a 5uH coil. I have worked into all of the western states (I am in Nevada), and have worked almost all states from here to Ohio using this antenna and a barefoot Yaesu FT-857 on 160m.

I use the same antenna - minus the extra coil - for 75m mobile and have been told that I have to be running an amplifier. I get 15-20db over S9 reports into Hawaii almost nightly on 75m with this setup.

The one thing that I cannot figure out is that it doesn't work no where as good on 40m as it does on 75m. I have no problem working mobile to mobile on 75m to about east Texas, but on 40m, even when conditions are good, it is hard to work fixed stations in Indiana; when I have no problem doing so on my inverted V @ 30ft.

John, KC8NTP, east central Nevada
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KE3WD
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Posts: 5689




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« Reply #1 on: September 05, 2007, 09:09:58 AM »

It may just be that the mobile antenna acts like a vertical with a lower angle of incidence, making your signal just overshoot the stateside stuff on 40.  

Here is a link to how our military uses the mobile vertical for NVIS operation, note the pic of how the tieback is implemented in the reverse of what most think, in effect turning the vertical plus vehicle body into a very low to the ground dipole:

http://www.athensarc.org/fm2418m.asp

We used this trick in the USAF years ago to do local comm nets across a few states, there will be hams coming along here arguing that it won't work, of course...

Vehicle is typically stationary when doing this one.  


KE3WD
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K0BG
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Posts: 9883


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« Reply #2 on: September 05, 2007, 09:26:19 AM »

I only have a mental picture of the antenna. Based on that, there are a few things to keep in mind. A cap hat does raise the radiation resistance. However, it needs to be as high as possible. Far too many folks mount it just about the coil which defeats its purpose, but adding loss to the coil. A basic rule of thumb is, at least twice the coil's length above the coil, and preferably at the very top of the whip (admittedly hard to do). If the cap hat is spoked, like DX Engineering unit is, electrically connecting the ends will almost double its effectiveness.

Tom Rauch, W8JI, has several articles about operating 160 mobile, cap hats, and other pertinent top band data. It might behoove you to plow through it all.

Alan, KØBG
www.k0bg.com
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KC8NTP
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Posts: 16




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« Reply #3 on: September 06, 2007, 12:22:59 AM »

Alan:

The cap hat that I use is made out of 3/16" copper tubing that is soldered to a fender washer. I have 14 gauge wire around the perimeter.

I know that having the cap hat higher will work better. However, this arrangement shows a major improvement on 75m - not necessarily signal. I have a much higher feed-point impedance and have to use less of the coil to tune it to 75m. Without the cap hat it is a real bear to tune on 160m.

I am just impressed that I can talk all over the country barefoot (mobile) on 160m/75m. However, the way that I am, I am always trying to improve what I have. Doing low bands mobile is all new to me. I know that the concepts are the same with any antenna system, but this has been an amazing adventure in antenna designing and modeling for me.

The other person said (sorry I forgot your call) that the radiation angle may be too low for working anything close in on 40m. I have thought this as well. The best signal reports that I get are in day light on a 600-700 mile path anything closer and it is weaker, and anything much farther and it starts to get weak. I have thought about running a horizontal wire off from the screwdriver  - taking the whip off - going to the back of my truck, and supported by a small mast. I think that this may give me some interesting NVIS while mobile on 40m, and it may work fairly well on 75m. Do you think that this is a sound idea?  I read about a setup like this for local 75m mobile work; local would be a misnomer - regional would be more appropriate.

I always value your opinion, Alan, as you helped me in the direction that I went with this antenna setup in the first place - that is, you told me to use inductance matching instead of capacitance matching, and gave me hints on how to make the antenna more efficient. I have also read many things that you have written concerning mobile H.F. and they have always been of great help.

Oh, by the way, Alan, I would also like to thank you for telling me a while back to use a halo type antenna for 6m mobile SSB work. I built a squalo (I am not going to buy one when they are a breeze to build), and within a week there was an opening and it worked great. I could not even hear anything on my vertical, but signals with the squalo were S3-S5. I know that this is unrelated to the post, Alan, but I had to thank you while it was still on my mind.

John, KC8NTP, east central Nevada
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K0BG
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« Reply #4 on: September 06, 2007, 12:17:50 PM »

Thanks for the kudos.

This whole NVIS thing is amusing to me. Lots of folks think a mobile vertical has too low a radiation to talk on say 40 meters close in. The real case is, band conditions are more of an indicator than the angle of radiation. Also, you have to understand we're at the low end of the sun spot cycle, and it effects all of the bands.

A common suggestion for NVIS is to fold the whip over so it's L shaped. This has very little effect of the radiation angle, although it does distort the donut as it were. True NVIS it isn't!

And speaking of 6 meters. Just yesterday, I caught a brief opening into Alabama. I could hear the station on the vertical, but the squalo was easily 20 dB better.

Alan, KØBG
www.k0bg.com
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KE3WD
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Posts: 5689




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« Reply #5 on: September 06, 2007, 05:26:18 PM »

Be amused all you like, Alan, it is exactly what we did for certain USAF comms, it is exactly what the US military still does, as referenced in my link above -- and it does indeed work better.  

Most people tie the whip down in the wrong direction, that link with pic shows the way to do it.  


But don't let actual multiple realworld experiences get in your way.  Pontificate!  


!


KE3WD
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