Low Band Transit Antenna?

[High_order1]

Yes, I am serious.

There would be a market for it.

They can make a 18" CB (27MHz) antenna. Why not something similar for 37?

-Shawn

[Grog]

Because it would suck

[High_order1]

But, why? Why would it have to suck? Technology has been pushing the edge of the physics behind propagation for years now. We've reaped the benefits in the other bands. Sure, it's a long wave. And no, I wouldn't expect to be able to talk to Canada. But there has to be a way to get a little range and sensitivity in a footprint that doesn't require a guy wire or replacing the aerial every 15-20 underpasses.


So, let me throw this bone out there for you all to pick apart. What would the propagation characteristics be if you took a full 1/4 wave radiator, put it in a coil former, and made a mattress spring out of it? Would inductance cause a major change in its' impedance? Does low band have to be horizontally polarized?

I am serious about this. I have turned it over in my head for some time now, but I am limited on horsepower when it comes to these kinds of calculations, and I don't feel like spending money on software so I can model the results. There are still a LOT of users in the 30 - 50 MHz band. I don't know of one that likes the whip. The only other thing I can find has a four foot vertical.

Well, at least I didn't get smoked as bad as I expected.....



-Shawn

[k2hz]

Someone, I think it was ASP, used to make a low band transit antenna which I have seen on a 44 MHz bus system until they moved up to UHF. I also used to maintain a system on 37MHz and ASP made a center loaded antenna just like you described that is what we used. It was about 2' high.

Performance was not the equivalent of a 1/4 wave whip but it worked fine for our application and the trucks were not back in the shop all the time with a torn off antenna like with a regular base loaded type. ASP discontinued this antenna after a few years. When we inquired about ordering more, they said we were about the only customer that ever bought that model so they were not going to make any more.

We had to resort to the trick of a 30MHz coil with a short whip but it was still longer than the center load and did not work as well.

[nmfire10]

They do exist. I have one on our rescue and one on our tanker. Next time I'm there, I'll get the make/model. They look like a plastic upside down white sauce pan.

[kf4sqb]

Yes, Shawn, as has already been said, it can be done. However, as has also already been said, performance would suck. Why? Because that's just the way things are. There is no way, period, to make a shortened antenna perform as well as a full size. Shortening an antenna is usually accomplished with a loading coil, and its called a loading coil with good reason. In essence, the coil, as the name implies, actually loads the antenna down, to make it appear, electrically, the same as a longer antenna. The load, like any other load, generates heat. The loading coil radiates very little usable RF energy, which of course lowers its efficiency. Kinda like putting a little 2' antenna on a CB: will it work? Sure it will. If you only want to talk about 2 miles down the road. Much like the little low-profile "hockey puck" antennas some people, like Antenex, make for VHF high-band. They work fairly well if you're in a place with good coverage. For someone like me, in a rural setting, they are praticaly useless. My best guess, for best possible performance coupled with maximum ruggedness, you would want a "blade type" antenna. They've been used by railroads for locomotive antennas, and in transit on busses and such, for years on VHF high-band and UHF. However, I've never seen or heard of one for low-band. My god, what a monster that would be! However, they are usually extremely rugged, and will survive repeated limb-bashing and such. All of them I have ever seen are made of fairly thick cast aluminum, and weigh a pretty good amount in spite of being made of aluminum.

As for your question about polarization, with a few caveats, it really doesn't matter in terms of performance. The main things to keep in mind are that all antennas within the same system need to be the same polarization (unless you're using it for "DX", which is a completely different animal from "local" comms), and most horizontally polarized antennas are at least somewhat directional.

I hope this answers some of your questions!

[nmfire10]

I should say that I really have no tested how far away these antennas we have will perform well. Including mutual aid calls, we are always within 15 miles of a tower with hilly terrain all around.

[Tom in D.C.]

If I can find my old Antenna Specialists catalog when I get home
next week I'll post the number of the 30/40 mHz bus antenna they
once made, if only as a reference point. As I recall they were of
the same general appearance as the VHF and UHF units with the
white radomes. My memory tells me the low, low band unit was
about three feet long.

[k2hz]

Low band is normally vertically polarized just like High Band and UHF and all normal 2way antennas are nominally vertically polarized. The transit antennas were about 3-4' long and were still considered to be vertically polarized even though the antenna appeared to be horizontal.

The 37 MHz center loaded antennas I mentioned worked fairly well. Range with Maxar mobiles was at least 15 miles mobile to base and 5-10 miles mobile to mobile in hilly terrain (upstate NY Finger Lakes area). This was compared to our our larger trucks that had 1/4 wave antennas and high power Mocom 70s that were good for 20-25 miles mobile to base and 10-15 mobile to mobile.

These short antennas mounted in the center roof of service vans gave a better match and less problems such as ignition noise pickup and directional characteristics than a 1/4 wave ball and spring on the side of the van. A regular base load antenna on the roof of the vans would not survive tree limbs and other obstructions, particularly in customer driveways.

When they were no longer available and we went to base coils designed for a lower frequiency to match a shorter whip (about 30") performance went down and breakage went way up. Drivers usually intentionally bent the whip horizontal or tied it down to minimize clearance problems and performance then did indeed "suck".

[High_order1]

Wow - been some good posts here. I look forward to seeing some of the information you guys are bringing. I also appreciate all the technical explanations, too.

I would really like to learn more about the center loaded antennae; the few places offering anything besides the ol' ball and spring don't mention them, now. I'm also researching rr and bigcity mass transit, as someone suggested, too.

Thanks again, and I hope everyone had a good turkey day - I am still reeling from the effects!


-Shawn

[mikegilbert]

I used a low band UnTenna on my suburban for a few years- it was a white dome about 8" high with a diameter of 12" Looked like a SatPhone antenna. It worked well- I might still have it somewhere. Their site seems to be down though:http://www.untenna.com

[bellersley]

I was speaking to a friend of mine who works for a company that does radio installs/repairs on utility vehicles that run on low-band up here. This is the antenna they use on a lot of their gear and he says it works just as good as the ball-mount they used to use:

http://www.sinclairtechnologies.com/cat ... SUF-DI.pdf

[KB2ZTX]

All of our counties in the CNY area use lowband, and we have had good success with the Untenna brand. Simplex to another truck is not always the greates, but to talk to our dispatch centers they work ok. They would not be my preference, but for some of our ambulances in our area, I'd be out there every other week replacing the mount and antenna (heck they rip a 1/4 800 mhz antenna out). I only had one minor issue with tuning one, I sent it to them and they shipped me a new one that week.

JAS

[High_order1]

he says it works just as good as the ball-mount they used to use:

Dang dude! 6 inches high, 6 inches wide, and almost 60 inches long.... I could rip off my luggage rack and put two up there cophased.... LOL

The untenna sounds like the way to go. Wonder how many hundreds of dollars one might cost? Their site was down, I did find older archived copies of it though. Zero on ebay, and I didn't find anyone selling one using a cursory net search. I'll start saving my pennies for the beginning of the year!

Tom - you ever find that catalog reference?

-Shawn

[pforbes]

RAB401xARA series from /\/\ and disguised lo-band car radio antennas from Comprod worked great for me. I even sold a disguised car radio/CB antenna to a fella with one of those early fibreglass minivans. He loved it!

[mikegilbert]

I'll sell you my untenna if I can ever find it.