View Full Version : 2m Antenna on Fiberglass RV
06-12-2007, 02:10 AM
I have a 2m mobile rig (Icom 2720h) that I would like to use in my Fiberglass RV trailer. I have a Comet Exceed 107B (no ground plane) antenna. What is the best way to instal this antenna on a fiberglass RV to get the best/strongest TRAN/REC signals. Compared to the Truck the signal strength in the RV is 1/3 the strength than what the signal is in the truck on max power. I suspect it is because the RV does not have a ground plane......anything I can do about it?
Although the information on one of the web pages I looked at seemed to indicate that the antenna was a half-wave design, and therefore would not require a ground plane, a couple of other pages indicated that on the 440 portion of the antenna acted like a half-wave design. The 144 Mhz antenna, according to the other pages IS a quarter-wavelength design, and therefore DOES need a ground plane in order to work properly!
I am curious as to how you are measuring the strength of the signal? Do you have a calibrated signal strength meter, or are you taking the S-meter reading from the radio attached to the antenna, or another radio some distance away? The reason I am asking this is to emphasize that S-meter readings in an FM application are almost meaningless because the IF stages are generally run at very high gain, and a signal has to be almost dying out before the meter will drop down. In other words, on most FM radios, the S-meter is almost full scale, or drops to nothing. YMMV !
Your groundplane could be as simple as some screening material taped down to the roof of the RV, or as elaborate as a completely different antenna. If you have tens of thousands of dollars invested in the RV, you shouldn't have too much concern about getting another antenna, making sure that it is a "ground-plane-less" design on 144Mhz! If the RV has been around the block, you might want to tape some screen material to the roof and mount your antenna on top of that!
Maybe others will have better ideas! Good luck. 73, Jim
I've routed the spare coax into a cloverleaf shape under the antenna and on top of the headliner. It'll improve a groundless 1/4 wave - a little. You have enough room to build a 4 stick counterpoise, screen - heck, even aluminum foil will improve things. If you have room and access above your headliner, there are lots of options.
You need something, your antenna's only 1/2 wave at 440. A screendoor groundplane does work - and it's usually spring before the neighbor misses it.
06-12-2007, 04:20 AM
KD8BXY, et al...
For sure, Jim's recommendation of a screen is a good one, as it relates to a groundplane...For 2m operation this would need to be at least 42 inches in diameter with the vertical located at the center for best omni-directional properties...The difficulty is in its installation between the underside of the Fiberglas and atop the headliner...The latter is often a challenge to remove and replace.
However, you might experiment with a metallic paint applied atop your Fiberglas...There are a few brands with high amounts of metal within the paint...The most obvious is the cold galvanizing paint sold in aerosol cans...The CRC brand is claimed to contain over 90 percent zinc in its formula...I've used it in a number of outdoor applications and it has been highly durable when exposed to the rain and high humidity here in Houston...There are aluminum metallic paints, some claimed to be "weatherproof" but I haven't any information about how much aluminum is actually in any of the formulas or how durable they are in an outdoor environment.
You need only an "X" pattern (equivalent to 4 radials) painted on your RV's top for decent performance into 2m or 440 repeaters using a 1/4 wave or 5/8 wave vertical...Again, each "radial" to be 21 inches long.
As another alternative, you can purchase aluminum tape...A thin layer of aluminum "foil" is vapor-applied to a Mylar or similar film and tenacious, pressure sensitive adhesive...Some varieties are very durable and weather resistant...It could be applied atop the Fiberglas and perhaps would work well as a ground plane when laid out in the "X" pattern under a 1/4-wave or 5/8-wave vertical.
06-13-2007, 12:31 AM
I was measuring the signal strength by the meter on the Icom 2720h. #The antenna is not mounted on top of the RV, but on the side near the top. #I've attached a picture of the homemade mounting bracket. #Thought about making a ground plane to mount on the bracket, but it would not be circular becasue of the mounting position. #Sounds like a 21 inch radius from the base would help...Oh and the roof is rubber too.
Correction to my first post: It is a Maldol antenna 1/2 wave on 144mhz and 5/8 on 440mhz
06-13-2007, 12:35 AM
Quote[/b] (KD8BXY @ June 12 2007,09:31)]I was measuring the signal strength by the meter on the Icom 2720h. #The antenna is not mounted on top of the RV, but on the side near the top. #I've attached a picture of the homemade mounting bracket. #Thought about making a ground plane to mount on the bracket, but it would not be circular becasue of the mounting position. #Sounds like a 21 inch radius from the base would help...
correction to my first post. #
Correction to my first post: It is a Maldol antenna 1/2 wave on 144mhz and 5/8 on 440mhz
I that a metal gutter just above the antenna mount?
06-13-2007, 01:02 AM
yes I believe that is an Aluminium rain gutter.
06-13-2007, 01:09 AM
Quote[/b] (KD8BXY @ June 12 2007,10:02)]yes I believe that is an Aluminium rain gutter.
I wonder if you run a short wire jumper from your mount to the gutter if that would help you out. I dont think it would hurt to try. GL 73, Sean http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif
Would you consider spending the extra $70 for one of the Comet 1/2 antennas that don't need a groundplane?
A resonant ground plane is going to do a better job than that alluminum rain channel, simply because it IS tuned, and the RF currents would be better ! Even a "short jumper" at those frequencies would have a significant reactance, and would limit the effectiveness of the attachment to the rain gutter.
However, Bruce's suggestion of the aluminum tape brought to mind the fact that where I used to work, we had some copper flashing TAPE that was no more than a few thousanths in thickness and was used to bind prototype circuit boards to a chassis to act as the RF ground in our SHF oscillator and amplifier circuits.
You might be able to find a source for the stuff yourself! Certainly an inch or two wide strip of that copper tape would be a very effective ground radial, and would be easy to solder to, etc.
Good idea, Bruce. Now, as to where you can find the stuff, "the rest of the exercise is left up to the student! "
06-13-2007, 01:30 AM
I've tried running a short piece if wire with alligator clips on each end from the mount to the rain gutter, with no luck.
06-13-2007, 01:38 AM
I thought the antenna I bought was the correct for my application. According to the print on the package. 1/2 wave on 144mhz and 5/8 wave on 430 mhz. If I could find a better antenna I may purchase it....
Something is wrong, the print on the package, or the description in the catalogs on the internet. Judging from your experience, I would say, the print on the package!
Sorry about that ! 73, Jim
06-13-2007, 02:08 AM
just to clarify I'm using a Maldol Exceed series ex-107b antenna and a Comet 3d5mc base. I'll look online again too.
One thing that bothers me about that picture, is that you have a good amount of the antenna BELOW the top of the RV. I don't know what material may be in the roof and above that rain channel, but it does look like that lower base coil is below the roof line. That could be part of the problem.
Good luck! Jim
06-13-2007, 02:14 AM
the specs I see match my packaging....
Reviews Summary for Maldol EX-107/EX-107B
#Reviews: 5 #Average rating: 5.0/5 #MSRP: $56.95 #
Description: Dual band (2m/70cm) mobile antenna with PL-259 base, coreless coil" AND "24K plated center conductor and coil, 1/2 wave on 2m, 2x5/8 wave on 70cm
The Maldol EX107B NMO dual band is a 2 meter and 440 MHz mobile antenna. It is a 1/2 wave shortening V-match on 2 meters providing 2.6dB gain and a 5/8 x 2 wave shortening V-match on 440 MHz providing 4.9 dB gain. VSWR 1.5 : 1 or less. This antenna can handle up to 100 watts
I could always try raising the antenna above the roof line... # appreciate eveyones help.
06-13-2007, 06:09 AM
Vertical Sleeved OCF dual band dipole doesn't need a ground plane
06-13-2007, 04:30 PM
I would think that your existing setup would work well, though I think the copper-flashing groundplane idea wouldn't hurt anything. ACE hardware carries copper tape and flashing.
I would look first at how you're comparing the signals. Are you using the same coax in both locations? When you say the signals are 3X stronger with the antenna on the truck, are you using the meter readings?
Depending on how the antenna is mounted, it may be somewhat directional. You may want to compare signals from different directions to identify what's going on.
In flat country, a half-wave antenna should perform well.
I assume the SWR is OK with the antenna mounted where it is?
06-14-2007, 12:41 AM
I wanted to thank everyone for their input on my problem. Turns out I found a bad ground on my coax. After a little solder we're back in service. http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif and good as new.