FT-7900 Transmit Problem

[dscott238]

I have a new FT-7900 with a quarter wave mag mount antenna. I recieve decently on 2 meters but I am told that there is lots of noise on transmit. I have had no luck on 70 cm at all. I have talked to some people ok but not consistently well. How do I go about troubleshooting this problem? Is the antenna not a good one? I must feed the line from the roof of the car through the door into the interior.

thanks,

.dscott

 

[kb0nly]

Get a better dualband antenna! A 1/4w cut for VHF isn't going to work worth a darn on UHF and vice versa.

Depending on what you have for a vehicle if you can install a lip mount on the truck or hood or something that would be your best bet, or go the extra mile and drill a hole for a NMO mount in the roof. Lots of options, but first thing you want to do is ditch the 1/4w!!

 

[wyomingmedic]

The problem may go deeper than just the 1/4 wave issue.

I would suspect either damage to the coax or the antenna itself is not making reliable contact.

Get somebody with an SWR meter capable of VHF frequencies and have a look. Visually inspect the coax (especially where it enters the door) and look for damage. Even having an oval or misshaped section can be a problem.

Lets start with those techniques.

WM

 

[K9WG]

I agree with WM. If you are using a true 1/4 wave check the continuity with an Ohm meter. It should be a short from the center conductor of the connector to the antenna, and open between the center conductor and the shield. Move the connector around while measuring to see if there is an intermittent short or open.

(for some 5/8 wave antennas there may be some resistance between the center conductor and the shield)

 

[kb0nly]

I think its merely the antenna, but the feedline and such are good things to check if you plan on using the same mount with a different antenna.

The bigger questions here... How distant are the repeaters your trying to work? Unless your close up to the repeater or people your trying to talk to get a better antenna and give the radio a break. Instead of running 50w into a 1/4w get an antenna with some gain and run less power and have more reliability. Using a 1/4w on one band is dumb enough, trying to run a 1/4w on a dual band radio is rediculous.

I'm not flaming the OP, just making a point. Your radio is only as good as the antenna you have it connected to. Where i live a 1/4w antenna ain't gonna get you jack unless your running 150w, and even then your not going to hear much coming back. Terrain and long distances to the repeaters is the biggest issue. I generally need to go 30-40 miles to talk into one of the local repeaters, and a few are farther out. Not a problem with 50w and my Comet SBB7 dual band antenna.

 

[kb0nly]

The problem may go deeper than just the 1/4 wave issue.

I would suspect either damage to the coax or the antenna itself is not making reliable contact.

Get somebody with an SWR meter capable of VHF frequencies and have a look. Visually inspect the coax (especially where it enters the door) and look for damage. Even having an oval or misshaped section can be a problem.

Lets start with those techniques.

WM

Your totally overlooking the fact that he has a 1/4w thats presumably cut for VHF and he is also trying to use it on UHF. SWR has to be horrid on the UHF side if the antenna was correctly cut for VHF. And we don't even know if it was cut to the right length. Too many variables.

 

[wyomingmedic]

Your totally overlooking the fact that he has a 1/4w thats presumably cut for VHF and he is also trying to use it on UHF. SWR has to be horrid

Actually it doesn't. I (and a lot of folks) use a 1/4 wave VHF antenna on the UHF ham frequencies as well. It is a 7/8ths wave and tunes up acceptably (about 1.8 to 1). It has an odd radiation pattern with some lobes that go off in all directions, but it is very much usable.

There was an article in a recent QST about how to add a hairpin match section to a 1/4 wave VHF antenna and help the radiation pattern on 70cm. It was elegant and simple, but not necessary.

So if the antenna is tuned for the VHF ham bands, it should tune up acceptably on the repeater portion of the 70 cm band.

As I said though, get an SWR meter.

WM

 

[kb0nly]

Actually it doesn't. I (and a lot of folks) use a 1/4 wave VHF antenna on the UHF ham frequencies as well. It is a 7/8ths wave and tunes up acceptably (about 1.8 to 1). It has an odd radiation pattern with some lobes that go off in all directions, but it is very much usable.

There was an article in a recent QST about how to add a hairpin match section to a 1/4 wave VHF antenna and help the radiation pattern on 70cm. It was elegant and simple, but not necessary.

So if the antenna is tuned for the VHF ham bands, it should tune up acceptably on the repeater portion of the 70 cm band.

As I said though, get an SWR meter.

WM

Yes, IF it was tuned correctly for the 2m band also in the first place but we don't even know that yet. I saw that article, i laughed.. Just plain stupid, why people would want such a compromise in an antenna is beyond me. Like i said, the radio is only as good as the antenna. I laugh when people buy a new radio and complain about its performance, then they try a better antenna and its night and day.

 

[K9WG]

... but I am told that there is lots of noise on transmit ...

I was thinking along the line of the noise. I guess the next question would be what kind of noise. Are talking about noise due to low signal strength, feedback or whining, or intermittent? I have seen (and had) many an installation where the receive has been good but the transmit bad (broken up) due to a broken center conductor or bad ground to the shield.

 

[kb0nly]

I was thinking along the line of the noise. I guess the next question would be what kind of noise. Are talking about noise due to low signal strength, feedback or whining, or intermittent? I have seen (and had) many an installation where the receive has been good but the transmit bad (broken up) due to a broken center conductor or bad ground to the shield.

Very good points also... A description of the noise would be most helpful.

 

[RadioDaze]

A couple of other things to check:
How good is the coupling between the magnet and the mounting surface? Thick paint? Plastic layer?
Is the coax damaged from having the door or window close upon it?
How is power supplied to the radio?

 

[kb2vxa]

"I was thinking along the line of the noise."

That was the thrust of the OP.

"I guess the next question would be what kind of noise."

That should have been the very FIRST question before all the nonsense started.

 

[kb0nly]

"I was thinking along the line of the noise."

That was the thrust of the OP.

"I guess the next question would be what kind of noise."

That should have been the very FIRST question before all the nonsense started.

True... But you cant help but ask "what the heck" when you hear someone asking about problems with 1/4w on a dualband radio.

 

[hill]

Actually ¼ wave antennas are very workable on single band radios, but not on a dual bander. Just look Federal Government vehicles that mostly operate on VHF High-Band and what do you see, but a ¼ wave antenna.

 

[dscott238]

I will get a better NMO antenna. This will also help with the cable. I'll check the final installation with the SWR meter. I didn't think to ask what kind of noise but I will follow up on that. Thanks for your help.

 

[wyomingmedic]

KB0NLY,

I see you live on the Minnesota side of the MN/SD border. From what I remember, the terrain is rather FLAT (My great grandmother still lives in Watertown ). Certainly a good use for a higher gain mobile antenna.

But living in the craggy peaks of the Rocky Mountains, I find an almost magical performance of the simple 1/4 wave. On the roof of my truck is a 5/8ths wave VHF antenna and a 1/4 wave VHF antenna (and several others). Both are hooked to 2 meter radios. I primarily use one as a scanner, but it serves as my backup when the other does not work.

Many many times when I am deep in a canyon or near a mountain, My 5/8ths antenna has too much of a compressed radiation pattern and I am unable to hit repeaters which may be 3 to 5 thousand feet straight above me. I can grab the radio hooked to the 1/4 wave and get in full quieting while the higher gained radio is incapable of getting through. The same can be said for simplex communication with huge variations in height between stations.

And was already pointed out, almost ALL government vehicles, ALL federal fire trucks and 99% of firefighting contractors in the Rocky Mountain region use 1/4 wave antennas. They simply work better in certain environments.

Flat terrain, 1/4 wave vs.a higher gain model, Sure, no contest. But many of us do not live in such areas.

73 and best wishes,

WM

 

[kb0nly]

Flat? Here? Not really... I mean flatter than the mountains yes.

Buffalo Ridge between me and the West, going to the west can be an issue. As for general hills i can't name one road around here where you don't dip down into valleys every couple miles, valleys that are deep enough to lose line of sight.

I can't argue the use of 1/4w antenna's, but for my experience they don't do crap for me. Those government vehicles driving aroung with them have one big advantage, generally 100-120w radios. Brute force makes up for antenna gain! LOL

 

[wyomingmedic]

I guess my thought on a valley is 3 thousand feet in a canyon. Like this

So maybe we are coming from 2 different schools

I do use my 5/8ths wave when away from the mountains. Hands down, beats the 1/4 wave in the flats.

And most of the government setups use the GMH or DMH Bendix King mobiles. 50 watts max. But the DB difference between 50 and 100 watts is negligible.

WM