FM transmitter antenna questions...

[Mikebits]

Certainly. Therefore, you should only be prepared to tolerate cable loss if you can replace it with antenna gain.A 2:1 VSWR does not cause a 2:1 loss of power. At worst, it could increase the cable loss. Repeating myself, the experiment will tell Roger if he's improved his situation or not. We haven't suggested any permanent changes.

Roger,
Let's assume that the board is properly grounded to the case. Just scrape the case where the connector passes through and use star washers on both sides to make good contact. Looking at your photo, I myself would drill a dedicated hole for the connector just to the right of the whip antenna's hole and run a short wire to the pad where the whip was. (You will want to remove the whip of course.)

Since the subjects of loss and impedance matching have been brought up, RG6 cable is widely avalable, is 75 ohms, and has much lower loss. Generally I use f-type connectors with RG6 to be compatible with my cable, tv, etc..

Okay, so my memory does not serve me so well about power loss, but mismatch was correct no?

 

[mneary]

The VSWR calculation needs to take the electrical cable length, loss, and final load into account.

In school I recall we had to use something called a Smith Chart.

 

[Mikebits]

Well I do believe I was in error about the power loss figure, but I am pretty certain a 25 ohm mismatch equates to a 2:1 VSWR. I recall this because we use to have a test station for transmitters where I use to work where the test was open load, 50 ohm load and 75 ohm load. The 25 ohm mistmatch would cause a 2:1 mismatch. I have no supporting calculations to support this claim.

On a smithchart, I think 25 ohm mistmatch lines up with 2:1 vswr.

 

[Mikebits]

Smith chart link.
**broken link removed**

 

[ericgibbs]

ref the 50R versus 75R, this pdf gives some info.

 

[audioguru]

If you turn up the RF gain control the output transistor might distort the RF causing interference on other frequencies (176.2MHz, 264.3MHz etc) but the sound will not be distorted because it is FM. You have no way to monitor the interference frequencies.

 

[51fordf2]

If you turn up the RF gain control the output transistor might distort the RF causing interference on other frequencies (176.2MHz, 264.3MHz etc) but the sound will not be distorted because it is FM. You have no way to monitor the interference frequencies.

That's the kind of stuff I am concerned about. I'm not sure how much that would affect my situation, living where I do, and the low power I am putting out, but I'd rather not, just in case. I'd rather err on the side of safe, than sorry.

Thanks all! I have a lot of things to try, and think about. Although I have never gotten involved with radio, I have the utmost respect for those that do!

Roger

 

[Sceadwian]

Ahh ford, it's always nice to see a mad scientist that has respect for his neighbors =) Most experimenters would just slap together any old circuit off the net and try to blast it as high as the could.

 

[51fordf2]

thanks! I stop at Stop signs also, even when no one is around. Dad was an old-school Radioman in the Navy for 27 years, when THEY had to fix their own gear. Even if I didn't learn radio, I DID learn respect...

R

 

[Sceadwian]

I'm the same way, especially when I'm driving after dark, strictly the speed limit observe all stop signs etc... Checking over my shoulder even if I know there isn't a person around for a mile =) Good habits to get into. My pops was a sonar tech in the Navy here. He got along pretty good with the radio guys =) Same discipline basically just different aspects.

 

[51fordf2]

Well, I am done! I used the BNC connector, and shortened the feed to about 7 feet. I also replaced the tubes on the dipole with full-wave. I put the freq counter on it, dead solid at 88.099/88.100, and I'm getting the longest range I have yet...oh yeah, turned the gain up, just quick test, no real gain, and put it back. Wasn't going to leave it, just nosey.

Thanks to all for the help, suggestions, and comments - I really appreciate it!!

Roger

 

[Sceadwian]

I'm taking notes =)