Batboard

I'd put off the idea of geeking out my SUV for a while now, but now I'm kicking the idea around again.

I've seen some hams get pretty good dual-band antennas (VHF/UHF), ones with fairly high gain. And then they mount them on the back of their SUV. Not on the roof, but by the rear hatch, so that it doesn't stick up above the roof. This has the great advantage of being compatible with garages and such.

Logic dictates that this would give an awful radiation pattern, and effectively prevent you from hearing / transmitting at all "in front" of you. But I see it done a lot, so I have to wonder if it's not as bad as I'm imagining it to be?

And which is better generally -- something high gain (4 and 6 dBi VHF/UHF) stuck behind an SUV, or a pigtail-style unity gain antenna on the roof?

My semi-educated guess answer to your question is that it would be
an equal trade off, or call it a tie, between the two situations. The
upper, exposed part of the gain antenna would probably do nearly the same
job as the center-roof mounted low/no gain antenna, but you'd most
likely still have some skewing of the pattern rather than the ideal
360 pattern you want. I've run 60 watts to a VHF/UHF dualbander on the roofs
of my last three Subaru Outbacks with great results, even on long hauls
out in the boonies. (The antennas cost all of $15 and even though
they're only about 14 inches tall they still manage to hit things now and
again but at that price who cares?) I also used to put on a 3DB gain VHF
antenna for "distance" when going down to our country place but I've
given that up, too; just not worth the bother and no measurable difference
between the big Larsen whip and the small "in town" antenna, plus I can
always work both bands. Now, if I did a lot, and I mean a LOT, of direct
146.52 simplex work mobile it might be a different story, but all of my
operating consists of repeaters or public service monitoring and the simplex
contacts are few and far between.

Cut a hole in the roof, mount an NMO, and be done with it. When trade-in
time comes you put in a hole plug and no one, based on my personal
experience, really cares.

are you against drilling a hole in the roof? they do make a low profile oil can type vhf that only sticks up about 5 inches? or does it have to be dual band?

"Oil can" VHF antennas have lousy transmit bandwidth, typically about
1 mHz. Same type antennas on UHF and 900 mHz have greater usable
transmit bandwidth, which means they stink for VHF but are okay for
the other two bands. On the observation side, there's gotta be a reason
you don't see more of them in use than you do.

Tom in D.C. wrote:Cut a hole in the roof, mount an NMO, and be done with it. When trade-in
time comes you put in a hole plug and no one, based on my personal
experience, really cares.

I'm not at all opposed to doing this, I just didn't want a 5' whip sticking up on top of an SUV.

In this case, bandwidth is going to be pretty important, as I'm looking to cover a fairly broad range.

Tom in DC,

What dual bander are you using that is only 14" and is that cheap? I am interested.....

Thanks,

Wes

Wes,

I have gotten a short dualbander from Antenna World in Florida on three
past occasions. Right now I'm using two of them on my car, one on a permanent
NMO for VHF and UHF, and an NMO mag mount for VHF P25 driven by a Thales
T25 HT into a 60-watt amplifier. Unfortunately, when I went to get you the URL today I found that their website is "up but inoperative" at this time. When I dealt with them in
the past they were very good at what they did and the stuff was priced right.
The unit I use is about 17 inches high with an open coil in the center, and
though it's been a while since I "Birded" one of them as I recall they were both
pretty flat over both ham bands. It's sold in two pieces, a base and a whip,
and the cost, again as best I can recall, was between $15 and $20 complete.

Regards,

Tom in D.C. wrote:Unfortunately, when I went to get you the URL today I found that their website is "up but inoperative" at this time.

Don't worry, we just have to wait until July 4.... 2007...

Tom in D.C. wrote:an NMO mag mount for VHF P25 driven by a Thales
T25 HT into a 60-watt amplifier.

Do tell. SMA to Mini-U adapter?

Another question:

Does the mission requirement say it *has* to be a dual band antenna? A seperate VHF-hi band and a UHF antenna could also be plexed together if you don't have seperate ports on the radio, and a VHF-hi antenna isn't all that tall at all.

Another thing you touched on but I don't think has been discussed, is that I think changing the antenna gain and feedpoint also changes the propagation characteristics of the antenna. I think a lot of that decision would depend on the environment where you live - flatlanders are better served many times by a different antenna than the mountain folks.


-Shawn

High_order1 wrote:Does the mission requirement say it *has* to be a dual band antenna?

Not at all. It's my POV. I don't want a porcupine, but two antennas could work.

I think a lot of that decision would depend on the environment where you live - flatlanders are better served many times by a different antenna than the mountain folks.

It's somewhat hilly terrain. Not mountains, but certainly not flat.