2M antenna tuning???

[crawler#976]

OK, after the sucess I had with my initial Yaesu FT-2800M I decided to put one in the beater. The first radio came with a 1/4 wave mag mount antenna.

http://www.universal-radio.com/CATALOG/hamantm/2141.html

When I ordered the second unit I decided to upgrade the Taco antenna to a 5/8 wave mag mount.

http://www.universal-radio.com/CATALOG/hamantm/4479.html

I expected to see an increase in performance - that's not the case...

Will proper tuning make the difference???

Will one of the $80.00 range 2M SWR meteres be sufficient???

Thanks for any input,


Mark

 

[HenryJ]

There is no substitute for a matched antenna.

I don't have more than a few years experience in the amateur bands, but I have done lots in our fleet trucks and on emergency vehicles. ( 150-158 mhz)

The lengths can vary quite a bit. The same antenna that is too short for one vehicle , might be too long for another. There is not a huge range , but probably up to an inch in a 1/4 wave whip.

These were all NMO mounted to the vehicles. Well grounded and centrally installed were possible.

Some time with a SWR meter would be well worth investing in. I would try find someone locally who has one and would be willing to help.
I found a used SWR meter on ebay. They are expensive, but I do quite a bit of tuning. I am sure a query on a local net would yeild some operators that would be more than willing to help you out. I know I would if you were closer.

 

[asteffes]

OK, after the sucess I had with my initial Yaesu FT-2800M I decided to put one in the beater. The first radio came with a 1/4 wave mag mount antenna.

http://www.universal-radio.com/CATALOG/hamantm/2141.html

When I ordered the second unit I decided to upgrade the Taco antenna to a 5/8 wave mag mount.

http://www.universal-radio.com/CATALOG/hamantm/4479.html

I expected to see an increase in performance - that's not the case...

Will proper tuning make the difference???

Will one of the $80.00 range 2M SWR meteres be sufficient???

Thanks for any input,


Mark

Have you tested reception and transmission quality at all power levels with both antennas? You might find some differences with certain combinations. FWIW, I went from a mag-mount rubber ducky 1/4 wave to a roofrack-mounted 5/8ths with little difference except at high power.

 

[DaveInDenver]

I expected to see an increase in performance - that's not the case...

Will proper tuning make the difference???

Will one of the $80.00 range 2M SWR meteres be sufficient???

The difference between 1/4 and 5/8 wavelength might be big, might not. Depends on what you're trying to achieve. A 1/4 wavelength works great in the city and in hilly terrain since it's got a uniform pattern in all directions, like a dome over you. The 5/8 has a little better range in azimuth at the expense of altitude. It's not completely deaf above you, but some lost range and sensitivity anyway.

Where it gets complex when you start looking into how the signal is launched. The size and shape of your ground plane is the primary factor here. I use the rule of thumb that the ground needs to extend out roughly the same length as the antenna is tall (there is an actual a reason for this, if you want to hear it). With an unloaded whip that is easy to figure out, if it's 2 feet tall, the roof needs to ideally extend 2' out in all directions from the whip. Yes, that's not always practical but a good goal anyway. A loaded (with a coil) whip has a smaller near field strength and probably can live with a smaller ground plane. Anyway, a 1/4 wavelength 2m whip is usually about 24" tall and a 5/8 wavelength whip is usually around 50". My point is it's next to impossible to get 4' in all directions around a 5/8 wavelength whip. So you generally have a compromise in at least one direction. Loaded whips can help, but the best combo IMHO is a 1/4 wavelength whip right in the middle of the roof. Others antennas are a compromise in some way, which isn't to say they /might/ be better in particular ways, but they are also giving up something elsewhere.

When you say increase in performance, how are you measuring this. Range to a repeater, for example? The difference needs to be checked both transmitting and recieving.

The short answer is yes, absolutely tuning makes a difference. Ham antennas are generally better than CB antennas in being closely tuned from the factory, but the assembly tech has to assume some things. Your counterpoise (ground plane) and feedline (coax, connectors, etc.) all affect the SWR and ultimately the efficiency of your antenna. So all antennas have to be finally tuned after being installed.

I have a MFJ-864 meter, which covers HF, VHF and UHF (all the bands I happen to have CB, 2m and 70 cm) and has worked fine for me. Any radio shop should be able to tune your antenna for you, although this might or might not be free. My guess is the people at the shop you buy your equipment would probably do it for you if you ask.

 

[crawler#976]

Thanks for the replies!

I was hoping to add more range to the rig - it's not uncommon for me to have to send 60 to 70 miles to hit a repeater. The 1/4 wave has worked well at those ranges with just a little bit of bacon sizzle...

I've purchased my stuff thru Universal Radio. Maybe I can find a local Elmer with a meter.

 

[ParadiseCruiser]

Will proper tuning make the difference???

Thanks for any input,

Mark

ABSOLUTELY x2. Unfortunately, proper tuning can be complicated, and simple tuning to attain the best SWR can often cause more problems than they solve.

I agree completely with Dave, but I'm going to repeat some of what he said in a slightly different way...

If you take an antenna - say a 1/4 wave whip - that is perfectly tuned (resonant) for a given frequency, and place it in a perfect location on your truck (in the middle of the roof), you will get get a perfect 1:1 SWR (in theory).

Move that same antenna to the front bumper (worst possible location for an antenna), and now the SWR is something like 4:1 or 5:1... because you have lost the counterpoise (ground), and you have introducted a ground effect (the earth) because the antenna is too close to the earth (for a 2m antenna).

If you trim this bumper-mounted antenna for a perfect SWR, all you have done really is hack it up: Now you have a poor counterpoise, poor ground effect, and a non-resonant antenna. Ever wonder why CB antennas mounted on the front bumper work like ******t?

Sure, the SWR looks good to the radio (and meter), but all you have done is cancelled out all of the poor SWR's, and done nothing to change the faults... and you will end up with a fraction of the effective radiated power (ERP) than you would have had in the first instance. This is a very common problem, made by newer licensees and older hams alike.

The only way to do this properly, is...

1. Use a good SWR meter (or borrow an antenna analyzer);

2. Tune a mag-mount whip in the center of the roof to get the antenna as close to resonance as possible at the desired frequency;

3. Move the tuned antenna around the vehicle to find a location as close as possible to the final mounting location where the SWR is acceptable (hopefully less than 2:1);

4. Retune the antenna for the best SWR possible.

Every truck will be different, so you cannot move a tuned antenna from one truck to another and expect the overall tuning to be same in both cases.

Then you have the matter of how 1/4 wave antennas differ from 5/8 wave antenna as Dave mentioned, coax length, power loss in the coax and connecters, etc. It's a science really, but getting the antenna location and tuning right - one of the most critical issues - will result in good performance.

Cheers, R -

 

[crawler#976]

Made a call for an Elmer on one of the heavily used local repeaters yesterday - have an opportunity to borrow an analyzer. Hopefully that will help show some improvement in the performance of the 5/8's wave setup.