VHF Radio Antenna Question

[Shelby Griggs]

I am looking to get a CB radio, and already own a business VHF radio on 151. 625. I have been looking at the FireStik antennas for both radios. The CB is no problem, but can someone shed some light on the 2 meter antennas for use with the VHF? I calculate them to be a little short, but I am basically ignorant of antennas and radios?



What I would like to do is use the FireStik fender mounts on both sides, one for the CB and one for the VHF. Any comments or help would be appreciated.



The FireStik antennas can be viewed here FireStik.



Shelby

 

[EMDDIESEL]

I have a radio that operates at almost the same frequency (150-151). I asked the radio guy at the Railroad shop about possibly using a firestick for it so i could have two matching antennas (One for a CB one for the RR radio). He said it cant be done judging from his expierience. What i would try is calling Firestick tommorow and asking them. Ask for a tech guy and he could probably answer your question. Tell him you already own 2 Firestick antennas so he does not just say yes with the hope you will buy 2 antennas from them. Good luck and let me just say again, I love you truck

 

[ETOBICOKE]

1/4 Wave Antenna

You can calculate the effective length otr the antenna required for this frequency, just divide 234 by the frequency in MHz and you will get the quarter wave antenna length in feet. This would work out to be 18. 5 inches at 151MHz. Firestick antennnas are 5/8 wave, made by winding a wire around a fiberglass whip, this is how they are able to make a compact antenna with a longer effective length.

I think the only way that you can end up with antennas that match in appearnce, but not height, is to find a staight fiberglass whip that looks similar and shorten it to the required length. At least this would alow you to use the same mounts for both antennas.

The only other thing I should pass on is that I have found the Firestick antennas rather narrow banded, not very good if you plan to go up or down stairs.



Neil

 

[SLightfoot]

Antennas

If the 2m antenna is tunable, you may be able to use it.



The 2m Antenna is going to be a little bit too long for use at 151MHz.

If you want to use the 2m antenna, you will need to cut it, which most antennas has provision for.



By the way, if you are looking for good performance, skip the wirewound deals, and buy a proper bottom loaded 5/8 wave, it's worth it.



Wound antennas just don't have the apertures that a straight whip does.



Shawn

 

[Tim Hayosh]

Re: 1/4 Wave Antenna

Originally posted by ETOBICOKE

You can calculate the effective length otr the antenna required for this frequency, just divide 234 by the frequency in MHz and you will get the quarter wave antenna length in feet. This would work out to be 18. 5 inches at 151MHz.



Neil

Neil, could you just check my math? I want the optimal antenna length for use at 151. 880MHz. Using the above formula, I came up with 3. 8517 feet (3' 10. 22"). This would be a 5/8 wave antenna, right?



Is there something "magical" about 5/8 as opposed to 3/4 or some other ratio?



One other thing, there has been some debate among other users of this band about the need for a "ground plane". I tried my rig without the ground plane, and reception seemed to be MUCH worse, while Tx remained about the same. Opinions?

 

[ETOBICOKE]

5/8 Wave Antenna

Basicly, a 1/4 wave antenna is actualy part of a half wave diapole, it uses the chassis as a ground plane, this is the most basic type of antenna. The reason that the 5/8 wave antenna is popular is that it provides more power gain over a 1/4 wave, power gain= more signal strength and transmitting distance. With only 4 watts available, a good antenna set up makes sense. The 5/8 wave is not a simple stick antenna it requires an impedence matching devise, usualy built into the unit, to work properly. The other claim made about the 5/8 wave antenna is that the doesn't require a ground plane. This seems to hold true for large base units like the Solarcon I Max, but I have found that the Firestick units can't be properly tuned (SWR) without a good ground plane.

The Antenna length calculation that you have made is correct, for the frequency given. There is no magic frequency at which you will suddenly no longer require a ground plane, rather there is a slow transition transition, generarly once you get into the 2 meter band, no ground plane is required, the trade off is that you need to be in line of sight to communicate. It never hurts to have a good ground, in many cases it will help reduce interference prolems, and allow weaker signals to be copied.



Neil

 

[SLightfoot]

Antenna ground planes

Two points:



1) A 5/8 wave antenna provides gain over a 1/4 antenna. You cannot just cut a piece of wire to 5/8 wavelength and expect it to work. They need impedance matching devices to get you to a nominal 50 ohm input impedance. Usually this comes in the form of a base loading coil.



2) A 5/8 wave antenna NEEDS a groundplane to operate. If you do not provide a groundplane, not only will the performance suck, but you will not have a nice 50 ohm match into the antenna. The only antenna you can get away with NO groundplane is a 1/2 wave antenna. 1/2 waves work very well in applications where you have a fiberglass body to mount on. The 1/2 wave does not have quite as much gain as a 5/8 wave, but the difference is not that big.



For mountainous terrain, you are actually better off with a 1/4 antenna, since the radiation angle is so much higher - 5/8 waves don't work as well in this application, since their power is focused into a narrow vertical field.



Shawn

 

[drives mopars]

try Larson antennas

larson should make a 150 mhz antenna



or call amateur electric supply in vegas



or talley electronics in i think burlingame ca

 

[Shelby Griggs]

Re: try Larson antennas

Originally posted by drives mopars

larson should make a 150 mhz antenna

I had a Larson mag mount (1/4 wave) on my old rig, was looking for something that I might be able to fender mount like the Firestiks. Thanks for the leads though.



Shelby

 

[JSchooley]

Two important factors for antennas are impedance match, which determines how much power actually get through the antenna, and pattern.



The pattern is the most important, assuming any kind of impedance match. It will be effected greatly by what the antenna is mounted to, the area and shape of the ground plane and any nearby objects. The main difference between 5/8 and 1/4 antennas is the lower elevation of the 5/8 which works better for mobile applications.



After a lifetime of studying antennas and RF propogation I can say uniquivically that it is a *****.



Best advice is to talk to several HAMs. A lot of them are pretty sharp and have the experience with what works. Good Luck.

 

[Shelby Griggs]

Thanks For Your Help

I got the following e-mail from FireStik.



We use our basic VHF design for 2-meter and a few others in the general vacinity. The antennas have about 6 MHz bandwidth (a lot!) so we could set one here at 153 MHz and cover 150 to 156 which is way more than you need. The antennas are about 45" long.

You'd have to order it direct from us. Figuer on $20 plus shipping for the antenna (can make in any color we have).



You can reach us at 1-800-528-8113 from 8 AM-4PM MST, Monday - Friday.

We accept Visa, MasterCard, American Express, and Discover.



Thank you for choosing Firestik and for your persistence in seeking out our quality products.



Any radio experts have any comments? It sure would be sweet to fender mount FireStiks for both the CB and VHF, they would both more or less match in style, and it is a nice clean install w/o drilling the roof



Shelby