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Author Topic: Using AM/FM factory mounting hole for 2M?  (Read 596 times)
KG4VJD
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Posts: 77




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« on: December 30, 2002, 03:51:45 PM »

Hi there,

Just came back from a vacation with a 2m/440 (Workman DB-1, unity/2.1dB) gutter mounted over my 92 Caravan with 20 ft of RG-58U running between antenna and transceiver.  The SWR is 1.7:1.  I experimented with or without connecting a ground wire to the car body and notice no difference in performance with the help of a SWR meter and a field strength meter.

I used this setup with an Icom HT.  The performance of this setup is OK.  I don't notice a whole lot of difference over my Icom 1443B base loaded rubber duck.

I'm ordering a 5/8 antenna but am very hesitate to mount it at the gutter because the total height will be very tall.  I'm sure I'll hit something eventually and break the antenna (probably the gutter too).  My thought is to use the existing AM/FM antenna mounting hole for the 5/8 antenna (I also have a Honda Accord with a 5/8 mounted at the trunk and giving much better performance).  The antenna has a 3/8" x 24 connection and I am wondering if there is any mount designed to replace the AM/FM antenna for this kind of antenna.  I did search the forum and internet and have been unable to find any information for this idea.  I don't want to install a glass mount antenna due to it's poor reputation.

I came across some ideas from this forum about the steel cross bar with a 45 degree extension at the back.  I thought about using this setup on a mobile Jpole which I can hide the 1/4 wave matching section inside a PVC pipe and use a 38" whip as the radiator.  But I'm sure my XYL doesn't like this idea (looks too heavy duty/high profile).  I'm working on my CW so 2m/440 will still be my primary bands to work on.  

Anyone has an insight on this?

73,
Alex KG4VJD
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K0BG
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« Reply #1 on: December 30, 2002, 08:13:17 PM »

There are several companies making AM/FM replacement antennas which also operate 2 meters. I'm not sure if there is anyone making a replacement that is both UHF and VHF, but I'd look at Antenna Specialists. They have a web site.

If you use a good NMO (Motorola) type mount, and use a coil spring, you'll probably be okay as long as you don't drive into your garage like Al Unser.

Personally, I do not like temp mounts and prefer to drill a hole and do things correctly. If you can't do the roof mount, then look at the cowl area for a suitable position.

Alan, KØBG
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N2MDB
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Posts: 14




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« Reply #2 on: December 31, 2002, 12:20:05 AM »

There are mounts specifically designed for fender/edge of hood mounting (they look like L brackets).

Also... A company called Sti-Co makes disguse antennas. They match it up to your year/model vehicle and it replaces your AM/FM antenna and looks just like the factory one (it uses a coupling box I believe). They are popular with undercover police cars.
I'm warning you though.. It's not cheap.

Check out http://www.sti-co.com/disquised.php

Matt N2MDB
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KG4VJD
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Posts: 77




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« Reply #3 on: January 02, 2003, 10:39:10 AM »

Thanks Alan and Matt for the input !

After some more research (actually spending an hour staring at the Caravan), I'm getting more hesitate to replace the AM/FM antenna with the 5/8.  I think there are more work involved than just finding another better location to mount the HAM antenna.  Also, budget is a concern so I have to go with pretty much what I have for now.

Therefore I have these plans:-

A) Move the gutter mount to the location right above the driver side door and run the coax straight down then inside through a rubber groumet at the door jam.  This will shorten the RG58 from 20ft to 10ft and reduce loss.  For the gutter mount, I will connect a ground wire to the gutter (drill a hole there and then touch up paint it back) giving it a better ground plane.  Keep using the same dual band antenna (about 17-18" tall).  I think this plan will give me the highest location of the antenna and the least coax loss.

B) If plan A doesn't work well, then I will "home brew" a fender mount (as Matt said, L bracket) and mount it on the left side between the engine hood and the left fender.  Then install a Hustler SF-2 ($15) 5/8 antenna.  This will maintain a lower total height and getting a more distorted radiating pattern.  But I would think the 3db gain will compensate this.

Please feel free to give your opinion !

73
Alex, KG4VJD
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K5LXP
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« Reply #4 on: January 03, 2003, 10:07:46 AM »

I would suggest abandoning the idea of a gutter mount for a variety of reasons.  One, they are not mechanically very sound.  They usually do more damage due to corrosion around the set screws and flexing fatigue than just drilling a hole.  After installing many hundreds of antennas in my commercial 2-way career, nothing beats a quarter wave NMO mount antenna in the center of the roof.  A 'ground wire connected to the gutter' does not constitute a ground plane, it's just a DC ground.  Your 5/8 wave antenna, while it may match '1:1' will likely have a distorted radiation pattern, giving you nulls in some directions.  A 1/4 wave whip is virtually indestructable, cheap and easy to install.  In many years of 2-way work I've never seen any demonstrable benefit to 5/8 wave antennas, they just break more often due to their length and cost a lot.  The top of a van makes a wonderful ground plane, and there's plenty of room for several antennas up there.

Mark K5LXP
Albuquerque, NM
k5lxp@arrl.net


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KG4VJD
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Posts: 77




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« Reply #5 on: January 03, 2003, 06:11:41 PM »

Thanks Mark for your reply !

Hm......This is definitely an alternative for me since the total height will not be too high.  Got to talk to my XYL about it.

73,
Alex KG4VJD
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