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Old 12-09-2009, 04:56 AM   #15
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Quote:
Originally Posted by w7ts View Post
I have not decided what to do about an HF antenna. I am concerned about the large aluminum object being close to a high Q mobile antenna. ......

However the instructions say to keep it 50 feet away from large metal objects. I am pretty sure that includes an Airstream.
What? I would think that you would WANT a "large metal object" such as a trailer and tow vehicle UNDER the loading coil to act as a ground plane. Otherwise much of your presious RF is wasted.

Of course you need to get the loading coil high enough so it clears the top if the trailer; otherwise it won't be anywhere near as effective. The problem with using a "Bugcatcher" type center-mounted coil vertical is that you have to be able to conveniently reach the coil to change the taps as you change bands. I use a fold-over mount that makes this chore much easier but I still need to stop and physically get out of the vehicle which sometimes is not very convenient or safe. Still, this arrangement is preferable to using a tuner which is much more convenient. However, when you use a matching network there are going to be losses.

Finally you would probably want to use a capacity hat of some kind to maximize your mobile antenna's effectivness and increased bandwidth, especially on 80/75 and 40 meters.

My good friend Neil K6SMF drives a mini-van all over the country going to hamfests and flea markets. He has an Outbacker smack in the middle of the roof on a mag mount. He works all kinds of DX on 17 and 20 meters.

Never forget that HALF of the antenna system is the part you don't see - your ground. This is the secret that goes back to the very eariest days of radio.

I apologize for any non-amateurs for this semi-technical discourse.

73,
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Old 12-09-2009, 10:18 AM   #16
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Hi Mike,
Most of what you said I agree with, but it does not pertain to what I was talking about. I am not going to attempt to find a way to mount a HF antenna on roof of an airstream trailer. That is the only way you are going to get the trailer shell to act as a ground plane. It is not the loading coil that has to be above the trailer, it is the base of the antenna. Unless you have modeled it, there is no telling what the pattern of your antenna looks like,but I would guess it is pretty strange.
The antenna I have does not benefit at all from a ground plane. It is a half wave, center fed, and capacitively loaded dipole. Did you look at the link in my post? I have had experience with HI-Q antennas in proximity to other metal objects. They don't work well in any sense of the word.
If you want to continue this discussion, please PM me and I will send you my email address.
Regards,
Ken
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Old 12-09-2009, 10:41 AM   #17
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Old 12-09-2009, 11:12 AM   #18
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An "Oldie" but Goodie - Webster Band Sapnner

If you've been a Ham long enough you may remember an HF Mobile
Antenna made by Webster (long since OOB) known as a "Band Spanner". It's basically a center loaded whip with a rather long center coil fed with a copper sprial tape from the bottom. The antenna is less than 1 1/2" in diameter, a good radiator (easy to match) but is pretty heavy. Does not work well on the bumper at speed w/o a good HD Spring mount.

I have intentions of putting mine on the AS for HF and than doing what B4WEI did for 2m (that is a slick install!!!). I have the same issue discussed earlier (how to get it above the Coach).

Has anybody seen a "Fold-over" mounted on the AS bumper or A-Frame?

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Old 12-09-2009, 12:14 PM   #19
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Ive given some thought to using the existing TV antenna for 2m/440...This spring Ill be redoing the AM/FM antenna. Would like make that spot available for something ham related...
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Old 12-09-2009, 12:43 PM   #20
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I have a bracket mounted on the side of my tower with a HF Mobile antenna with a quick release and break over for easy changing of the coils that I use for vertical operations here are home. I have not tried this as of yet but I am considering modifying that mount to work on the AS.
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Old 12-13-2009, 01:28 PM   #21
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Quote:
Originally Posted by w7ts View Post
It is not the loading coil that has to be above the trailer, it is the base of the antenna.
Hello Ken:

What you state is true. Mobile antennas are always a compromise. However it is not often practical to achieve this especially with something as high as an Airstream.

Unless you use a "screwdriver" motorized antenna you will need to have easy access to the antenna if you want to change bands. You could use a 102" whip with an antenna tuner but this would not be ideal due to losses.

K6SMF mounts his Outbacker on the roof of his minivan using a heavy duty mag-mount so he can access it easily to change the taps. Outbackers are light and somewhat aerodynamic so I think that this would be the way to go when hauling a trailer.

Mounting any antenna on the roof of an Airstream or similar trailer would certainly exceed the height limit of 13' 6" which is not a good thing. My "Bugcatcher" is 13' 0" high above the pavement which is pushing it.

I own a "Buddipole" portable antenna which is a good solution for use when parked in a campground. There is also a tripod available for the "Outbacker" for use under similar conditions. I also have a Collins TD-1 tape dipole antenna which is also excellent when travelling. Suitable supports are of course required and they are not always available. Additionally you have to pull it down and change the length if you want to change bands or use a tuner.

Since I sometimes use a vacuum-tube based radio (KWM-2A) with a pi network it is far more forgiving of high SWR than a solid-state final PA. I intend on mounting it inside my 1968 Airstream which I feel is appropriate. However my overall mobile rig these days is an ICOM IC-7000.

My reason for posting this thread is simply to help other amateurs learn more about mobile antennas and to try to determine what is most practical for use when hauling a trailer. I have been operating HF mobile for 40+ years and used everything from the aforementioned Webster "Bandspanner" to the Newtronics "Hustler" to "Hamsticks. I find I get the best results overall using one of Henry Allen's "Texas Bugcatchers" for the past 15 years. I read with some sadness recently that Henry has ceased production of his wonderful antennas.

73,
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Old 12-13-2009, 01:38 PM   #22
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Unfortunately my AS doesn't appear to have a spare tire under it.

Any suggestions?

Thanks

Tom Bray
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There is an Airstream spare tire carrier, that stores the spare, underneath the floor, at the front end.

Andy
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Old 12-14-2009, 06:05 PM   #23
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Andy ... When I order new axles, I'll ask you about the tire carrier ...
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Old 12-14-2009, 10:40 PM   #24
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This is the system for contesting that I am slowly putting together. I have 6M, 2M, 70CM and 23CM antennas for it:
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Old 12-15-2009, 07:52 PM   #25
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I don't quite know what is going on but one computer shows the picture, the other doesn't. So here is another try at the photo:
Attached Thumbnails
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Old 12-16-2009, 02:51 PM   #26
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I'm running an Icom 706 to a classic old screwdriver (DK3). Radio and separate small sealed battery reside in a void under the starboard side closet, with remote face velcro'd onto the bedroom wall, along with "up/down" screwdriver control switch.
Coax and control wire runs under bed into rear compartment.
The wiring comes out of the compartment and the door still closes over it (no holes).
The DK3 rides on a 6 foot mast coupled to my rear hitch receiver.
Set up is satisfactory and no holes were drilled. The hitch receiver has sported "cool" inserts too, although my last one got beat up and I'm on the lookout for a new one.
Creative ideas on this thread.
73 de KF6LOP
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