Fm transmitters

I used an FM25 for a short while to simulcast what I was broadcasting on by AM station. The results were alright--with a properly grounded power supply and a homebrew dipole antenna I was able to receive the signal on 101.5 in Detroit (near many other stations--it's a pretty packed radio market) for about a half-mile in any direction. It's a good way to start messing around with FM without spending too much money!

Michael P. Ryan

Staying withint the realm of legal power, half a mile is too far. I was getting almost half a mile on a homebrew dipole, but on the official Ramsey dipole that is supposed to keep you legal, range was pretty poor. If you want to cover more than your own yard I really can't suggest part 15 FM. As far as a good signal meter, I never found anything either, the signal is just far to low for most things to measure accurately.

How do I figure a 1/4 wave for 89.1 mhz and can I use RG-6 to conect my ant. to my tx.
Thanks,
Brian
underdog radio

I use a FM-25A and it works to my purpose for transmitting around my house. The 25 apparently is sensitive to RF which produces hum and you can read more about this on the Ramsey Forum. I have built 3 FM-25As and all had hum which was eliminated by mounting them in a metal box. Considering that you will probably have to do this, I would look at a Ramsey model which is already in a metal box.

I use the supplied whip antenna and shortened it to 4 inches to limit the range to my home. Fully extended and transmitting from my basement the car radio range was about .15 miles listenable signal. I intentionally limit my range to just my house for legal as well as it is not my intent to broadcast to anyone but myself.

The audio on my unit is very good, but I did have to apply a software 15kHz. low pass filter to reduce artifacts due to high frequencies interfering with the 19 kHz. pilot.

Hope this helps.

Neil

Brian,

The FM-25A model I have does not care what the antenna is. I have run it from short to open with no ill effects. This may not be true for the FM-25B or other transmitters.

Which coax to use raises some questions. I recommend that if you want to build your own antenna that you consider a 1/4 ground plane (do a web search). They are easy to build and if cut to the proper dimensions, work first time with no need for tuning. Another nice antenna is a twin lead jpole but it requires a bit of tweaking and if your transmitter won't tolerate a severe mismatch, you could have damage while setting up the antenna.

The 1/4 wave antenna will present a load impedance of about 50 ohms, so a 50 ohm cable will give the best match. RG6 is 72 ohms (if I recall correctly) and will also work though the SWR will be about 1.5:1. My experience with many VHF ham transmitters has been that less than 2:1 is nothing to worry about.

Neil

The standard calculation for a quarter wave radiator is 234 / Freq = length in feet. So for 89.1 meg, 2.626262 (etc ) ft or about about 31.51 inches for a 1/4 wavelength leg. This should normally get you close but isn't always exact. Differences in conductor types, conductor diameters, etc can alter the exact length a bit.

AJ

First answer - it's not a good idea to use RG-6, or any other coax, to connect your antenna to your Part 15 FM transmitter. Coax consumes a small amount of your transmitter power in conducting your signal to your antenna (called attenuation), and low power transmitters don't have any RF power to spare. For best range your antenna should be connected directly to your transmitter and the whole assembly placed high and in the clear. You can run power and audio to the transmitter/antenna via cable as the power levels for your audio and power are much easier to manage.

Bluezplayer AJ makes a great point, and here are a few more points to add.

Another formula, which gives the same result but is a little more manageable for high frequencies, is:

11811/ Freq in MHz = Wave length in inches (NOT quarter wave, BTW)

11811/89.1 = 132.5589 inches

For a 1/2 wave multiply times 0.5, for 5/8s wave multiply times 0.625 and for 1/4 wave multiply times 0.25

132.5589 X 0.25 = 33.1397

One way to anticipate some of the variances Bluezplayer AJ refers to is to calculate what hams call the velocity factor, or the consequences of skin effect, caused by the fact that the RF only travels over the outside of the conductor, not through the mass of the material.

There is more information here:

http://www.jenving.se/tables.htm

For small wire radiators the standard approach is to use 0.95, so

33.1397 X 0.95 = 31.4827 or 31.5 inches as a rounded off first effort.

Ideally, you would start with a 33 inch element with the 31.5 inch measurement marked and then trim the radiators while watching your SWR meter and trimming back as the SWR went down. If you were using concentric tube or a screw adjustment for the final couple of inches you could really test the SWR and dial it in very close.

Experimental broadcasting for a better tomorrow!

SCWIS wrote: First answer - it's not a good idea to use RG-6, or any other coax, to connect your antenna to your Part 15 FM transmitter. Coax consumes a small amount of your transmitter power in conducting your signal to your antenna (called attenuation), and low power transmitters don't have any RF power to spare. For best range your antenna should be connected directly to your transmitter and the whole assembly placed high and in the clear. You can run power and audio to the transmitter/antenna via cable as the power levels for your audio and power are much easier to manage.

Just to note that the loss in a coax transmission line between the transmitter and the antenna is unimportant to legal Part 15 FM "coverage" as long as the transmitter can supply enough power to the antenna to produce the 250 µV/m field that is permitted three meters away from the antenna. Virtually every "Part 15" FM transmitter in use today is rated for more than enough output power for a legal Part 15 FM station even with more than 50% of its output power lost in the transmission line (even 90%).
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