School me on mobile antennas (1 Viewer)

[Tennessee Jed]

I'm studying for my license now and am looking at my mobile set up in the cruiser. Something that jumps out at me is the difference in mobile antennas between what you see Australians running on their trucks, and what is used here in the states. The Ozzies have these massive antennas, like fiberglass broom handles, and all I can find here are basically either little pieces of wire, or they are these bizarre "ham sexy" home-made looking contraptions made out of copper pipe.

Why don't we use the apparently really durable types of antennas they use in Australia? Are their antennas not correct for the frequencies we have to use here? (I think they mostly use 477 mhz?) Do the companies that make heavy duty antennas not make them for American use? Reviewers here will talk about a certain mobile antenna and how they're "amazed by its durability" when it's just a 2' piece of wire on a magnet base.

It's a total noob question, but there it is. Living in the southeast, treelimbs will rip the mirrors off your truck - a small antenna isn't going to last long.

 

[izzyandsue]

Antennas are based on the wavelength of the signal, so a 440 Mhz is about 70cm or 27.5 inches. Most antennas are 1/2 or 1/4 wavelength, so 13.7 in long works. A 2 meter antenna (140Mhz range) is under 40 inches. The higher the frequency, the smaller the antenna used. Like a cell phone, antenna fits in the phone and connects for miles.
The long antennas you see on rigs, 8 footers, are typically for CB, which are in the 26-27Mhz range and 1/4 wave is 8 ft or so. Or perhaps they have a regular antenna and use the pvc "boom" to mount their antenna as high as possible, since that does matter on a truck.
High frequency antennas can be higher if you have a radio capable of that in your truck, but you will need the General class license to transmit on it.
For the Southeast, I have this antenna https://smile.amazon.com/gp/product/B003H3DTGE/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 monted on the fender of my truck. As you know, trails are tight, lots of trees and the antenna needs to be flexible and a roof mount will likely not survive. I do have another antenna mounted on top of the roof rack but remove it when entering trails. Roof rack location is great for highway as it is the highest point. Once I reach a campsite for the night, I may put that antenna back up, but need to remember to remove it before continuing the trip.

 

[lugueto]

Durability is not a factor here. A flexible, little piece of wire is actually extremely durable.

The antennas used by the Aussies are tuned for the 477MHz UHF CB band.

This band, which is outside of the 70cm Ham band, is unique to Australia. So yeah, they basically make them exclusively for Australia and New Zealand.

Owning a couple of antennas is not uncommon. Having a couple of mounting locations is less common, but serves the same purpose. Change the antenna or location according to the terrain you're travelling on and you'll be fine.

Don't worry so much about the antenna you're running. Be careful if you're running roof-mounted antennas when under heavy cover, or when entering covered car parks. Other than that, you'll be fine.

 

[Tennessee Jed]

Thanks very much. That makes sense.

 

[Skidoo]

More likely those antennas are for 10-20 Meter band (14-28MHz) as the lower the frequency the longer the antenna.
While the States has pretty good repeater coverage for most remote areas "guessing" Australia has less coverage.
So 2M with repeaters works well in the States, down under they take advantage of the lower frequencies skip properties to communicate long distances.

 

[Kofoed]

Living in the southeast, treelimbs will rip the mirrors off your truck - a small antenna isn't going to last long.

If trees will be ripping your antenna off (or damaging the mounting surface) go 1/4 wave rubber ducky

 

[KliersLC]

I agree with the rubber ducky comment. You may be less than thrilled with the performance if you want to hit repeaters from further out. The comet csb series has a nifty fold over hinge so you can put it in the stowed position when in the trail. Another option is a mag mount. Up top for everyday use, center of the hood for trail use. Good luck!

 

[dmjay]

Just to set the record straight, in OZ, we have two CB's. The same 27Mhz AM/SSB
as the states and 477Mhz FM. We also have a vast repeater network, i can talk to somebody 100km away from my kitchen on a handheld CB.
The fiberglass "broomstick" antennas are a colinear high gain (6-9dBi) stick.
They work very well.
Cheers

 

[Tennessee Jed]

And would those colinear high gain antennas work in the US for ham use?

 

[dmjay]

Yes, but only on 70cm.

 

[Tennessee Jed]

Thanks very much. Trying to piece it all together. There's so much information out there that you don't need, sometimes so little that you actually do.

 

[Dragos80]

It all depends on the wavelength of the antenna.
A 1/4 wavelength 2m antenna is about 3/4 wavelength 70cm antenna.
You will see many radio manufacturers pair the two bands in one radio and many antenna manufacturers have a dual band antenna (2m/70cm).

If that Australian broomstick is the correct (pairing) wavelength it should work fine on 2m band.
The question is, why would you want to have and obtrusive object in your field of view? It could also be distractive for the driver, in my opinion.

 

[dmjay]

As i said above, the aussie broomstick is for UHF CB 477 Mhz. It will work on the 70cm ham band ( 430-440Mhz ) but the SWR will be out.
The 2m ham band is VHF 144-146Mhz.
See the problem?

 

[Dragos80]

Is this said antenna adjustable? If yes, then what is the problem in using US 70cm radio with it?
What is the wavelength of that antenna anyway? We can stop right there.

 

[vabluefish]

In regards to antenna length and off-roading, should longer antennas be avoided due to the constant banging around? I just bought a 38' and I noticed in the instructions it said not to use an antenna longer than 45" for off-roading due to vibrations and what not.

Excluding catching on branches, has that ever been a problem? Should I just buy a short rubber antenna for trail runs?

Also, when you guys remove the antenna for example, when you're leaving it in a parking lot for a few days and don't want anyone to rip it off, do you cover the mount connection with anything?

 

[wapitimud]

In the military, and in maritime boat use, they have some impressive long fibreglass whips for ~50MHZ and ~130MHz. In many cases, the active metallic portion of the antenna is only inside the top of the fiberglass whip, and may extend only a few feet. The rest of the fiberglass just gets it up higher.

Antenna performance is usually measured in dB gain, either as gain over a dipole, or directional gain. Your ground plane on most antennas is important. While it may look cool to put antennas on your front bumper, sometimes the best place can be the centre of the cab roof, depending on installation and physical size and wavelength of antenna.