Antenna gain question

[James86004]

As I peruse the vehicle mounted antennas for 2m/70cm, I see the antenna gain in dB over a dipole. I would think bigger numbers are good, but there are so many unity gain ones that I was wondering if there is an advantage to that?

 

[SunTzuNephew]

Not really.

low (unity) gain antennas tend to be somewhat wider bandwidth, but thats about it. I prefer high-gain antennas myself.

 

[gary in ohio]

The first thing you learn about antenna gain figures from manufactures is that they are lies. Without knowing the testing methods gain can mean anything. Are the gain figures real world or calculated possible gains based on antennas design? Where they real world conditions on a read car or in a lock chamber.

Most 1/4 wave single band mobile antennas are near 0db gain and most dual band mobile antennas will give you 1.5-3db on vhf and 3-6db on UHF.

 

[James86004]

The first thing you learn about antenna gain figures from manufactures is that they are lies.

Can you use their gain figures to compare antennas within a particular manufacturer's range? Or are they too inaccurate even for that?

Most 1/4 wave single band mobile antennas are near 0db gain and most dual band mobile antennas will give you 1.5-3db on vhf and 3-6db on UHF.

It seems like the 5/8 wave ones have more gain than the 1/4 - I am thinking the extra length is a worthwhile tradeoff for me if that is so.

 

[xtatik]

Can you use their gain figures to compare antennas within a particular manufacturer's range? Or are they too inaccurate even for that?



It seems like the 5/8 wave ones have more gain than the 1/4 - I am thinking the extra length is a worthwhile tradeoff for me if that is so.

I think the point Gary is making is that you're better off ignoring the mfr'rs fluff and instead, rely on basic antenna theory. There are tons of bogus products and claims being made, especially in the realm of antennas. The closer an antenna is to full wave resonance , the better it will perform in nearly all aspects. Keep in mind what you're installing on a vehicle is always just half of an antenna....and, this is true of all verticals. Think of a vertical antenna as being half of a dipole turned on end vertically. The other half of the dipole is your vehicle. So, the vehicle type, size, and the mounting position will all play a role in the antennas performance.

 

[gary in ohio]

The gain figures are just part of equation. You can use the gain figures, just dont take them as the only thing that you select an antenna from.. 2 5/8 wave antennas are going to have every similar gain. If you want a good VHF antennas then a G7 hustler its 5/8 over 5/8 about 7ft tall, GREAT single band VHF antennas but can be an issue mounting it on most vehicles.

Reviews, both technical 3rd party as well as operational reviews should always be taken into consideration..

 

[Charlie56]

I was at a ham shop over the weekend looking for a replacement 2m-70cm unity. The gain claims of one manufacturer over another for what looked to be the exact same configuration were confusing to say the least. Finally the salesman just said. Don't get taken in by the manufacturer's claim, just go by "bigger is better". So I replaced my 12" high unity with a 40". I am happy, --- for the moment.

 

[7wt]

I was talking to the guys at the local shack up here and the all prefer the lower gain antennas on 2M/70CM. The reason why is the higher gain antennas concentrate the signal lower to the horizon but a a greater range, this effectively aims the signal straight into the hills and mountains. A lower gain antenna with it's higher angular signal is more effective in the terrain up here. I don't know but it sounds good.

 

[gary in ohio]

Going from a "20" inch antenna to a "40" inch antenna is going from 1/4 wave to a 5/8 wave antenna.. So Gain will be higher between those two antennas. Going between "20" inch antennas you will see little difference in gains and going between various "40" inch antennas you will see little difference in gain.

Going to dual band had complexity to antennas sizes and the big issue with dual band antennas is the reliability of the coil. Some are open coils, some are closed. Open coils are more flexibility but can get gunked up with dirt, mud and ice. closed coils are less flexible but are not affected by the elements.

 

[James86004]

Are the closed coils those black cylinders I see partway up the antenna?

 

[BigJimCruising]

As has been said don't take the db gain figures as fact. Think of them more as a guide. Yes most of those antennas will have some gain to them. But there are so many ways of installing an antenna that will affect it's performance that you're better off making sure you mount the antenna to get the most effect instead of comparing db's of gain. Also you'll be better served by choosing the best antenna for your needs over it's spec's. For example, if you do a lot of off roading in dense forests choose an antenna that will be flexable instead of a tall rigid design.

 

[gary in ohio]

Are the closed coils those black cylinders I see partway up the antenna?

Yes, If you see a BUMP in mobile antennas its usually some kind of coil. Be it at the base or in the middle.

 

[xtatik]

I was talking to the guys at the local shack up here and the all prefer the lower gain antennas on 2M/70CM. The reason why is the higher gain antennas concentrate the signal lower to the horizon but a a greater range, this effectively aims the signal straight into the hills and mountains. A lower gain antenna with it's higher angular signal is more effective in the terrain up here. I don't know but it sounds good.

Actually, gain and resonant wavelength have little to do with "take-off" angles. The effective couterpoise (your vehicle and mounting position in this case) has more effect on take-off angles. Speaking in general, verticals will have low take-off angles if properly coupled either to highly conductive soil and/or a radial field (if ground mounted, this becomes the other end of the "dipole" I mentioned earlier). When mounted in an elevated position above conductive Earth (as on a vehicle) the radials or counterpoise (the vehicle)should be resonant to the antenna. For fixed antennas this is easier to accomplish, for a mobile installation a compromise is usually the best that can be accomplished.
In a fixed elevated installation (non-mobile) the radials must be resonant because they aren't coupled to conductive Earth, and ideally they should be elevated and be sloped downward and away from the vertical antennas base. The elevated and downward angled radials will reduce the take-off angle toward the horizon which is ideal for DX (distance) communication. So, as you can see, trying to accomplish this on a vehicle is going to be tough. As for the short distances involved in 2m FM communications.......I doubt you'd see much benefit in trying to maximize or minimize take-off angles. More importantly would be to mount these antennas in such a way that evenly distributes the reflected lobes (radiation pattern) of your signal by providing as balanced a counterpoise as possible. This is usually accomplished by mounting as high as possible and as centrally located over the vehicle as possible.