FM transmitter antenna questions...

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Glad to hear you got an increase in range already.

Bypassing the RC network can cause mismatch, that's why I suggest making the resistor smaller.

Yes, the circuit ground, case, shield, etc. should all be common - I suspect if you check the copper "pours" they will all lead back to ground. (otherwise, they would all just be a load of parasitic capacitances...)

Yes, a dipole will add gain - but you are already part way to that. Your case is the other half of the dipole, whether you like it or not. Sorry! That's just how these kinds of FM transmitters are usually designed. Tank and a transistor, nothing fancy, L7 is probably the coil on the tank and if it doesn't already have a lead grounded it will have a path to ground through the transistor that drives it.

I'm thinking you probably want to add the radials to the case itself for the lower half of the dipole. If that hole's so small it could touch you definitely want to drill it out bigger.
 
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Your case is the other half of the dipole, whether you like it or not.
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This not correct. The case acts a ground plane so the signal can push against it and drive "up and out" the antenna.

If you put a dipole on the transmitter you will lose range. This is because the dipole will most likely lay horizontal, like your arms stretched out at your sides.
The main antenna in a walkman is the lead to the earphones and the two will be at 90 degrees.
The hole in the case will make no effect at all.
The only reason for insulation is to allow the antenna to be connected to something and if the "something," is metal. the insulation of the antenna will be lost.
The thicker the lead, the better.

Secondly, a dipole will contain "feeder" and this consists of signal that are "out-of-phase" and "rubbing against each other." You will get loses in the feeder and nothing will be gained.

The only thing you will gain is: "directionability." The signal out the front and out the back will be greater than the signal out the sides.
 
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Actually, my dipole is vertical - it goes out the window, then straight to the corner of the house, lined up with the outer corner, where it's vertical.

So, is there a way to run outside, with the antenna? I mean, that still keeps my length correct (full wave, 10 1/2 feet?) without loss? So far I have just used a 1/2 wave, 5' 3-5/8"...but would like to try the full wave. How about a Delta, with one end to the ground, the other to the antenna output?

as for the hole, it's so close, that play in the antenna standoff can allow it to touch...

R
 
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I am still trying to get a grip on how a 300uw line of sight signal could travel 300 miles. Even with diffraction the path loss would knock it into the noise floor.

Path loss at VHF, UHF and Microwave Frequencies
 
I'm not sure what colin is talking about, but even under the most ideal of conditions his range estimates are... generous to say the least.
 
Actually, my dipole is vertical
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You don't have a dipole.

Keep the antenna away from any brickwork as you may have aluminium insulation inside the wall.

Just use hook-up flex and make it half-wave. The whole thing is very loose and almost anything will work provided the antenna is vertical and not near any metal reinforcement or cladding.

I don't want to tell you what I really get with 30mW as the range is astounding. With a turn-the-dial radio in a car, it can be picked up more than 2 - 3 km down the road and over all sorts of single storey houses. I have sold over 100,000 FM bugs ranging from 20mW up to about 90mW. This is total power consumption so the output would be less than 30% of this.
 
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As quoted throughout the thread, I know that the existing antenna is a whip. I have a dipole. It is constructed of RG58, approximately 20 foot long, with two copper tubes, each 5' 7-13/16" long, insulated from each other pointing straight out, one connected to the shield, the other to the center conductor, also insulated, about 1/2" apart on a piece of FP4. In my book, that's a half-wave dipole at 88.1 Mhz.

Isn't it??

My question was, and is, can I modify the transmitter to use the dipole?

Thanks,

Roger

R
 
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Mikebits said:
That 20 foot of coax might add quite a bit of loss to your xmit signal.
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I can shorten it a bit, but would like to keep it outside. If I move it inside, I can shorten it. Of course, inside, I might as well just keep the full-wave or half-wave single wire...

Thanks

R
 
i notice a control for rf gain next to the antenna input, have you tried turn it up? Also you could mount a uhf N type connector on the case and get a good piece of coax like 9913 to go to your antenna. Raise the antenna up enough to get the antenna clear of nearby obstructions and you should do much better on range, if not then build a small amplifier that puts out a watt or two.I might add that this is probably against the law.
 
As I'm not too familiar with RF, didn't want to do that, not knowing if I should or not...so didn't! )

I do know that you can over-drive the signal, and didn't want to do that. But, I guess experimentation is possible...there's another pot, can't remember what it was...I'll have to open it back up, and check...

The law is fairly vague - you aren't supposed to broadcast past 200 feet, I guess, but you especially aren't allowed to cause interference with anyone or anything, and if you do, THAT'S when the FCC gets involved, someone complaining. I live in a very small town, only a few stations any where close. Don't think I'd have a problem, even with a bigger signal...

Can I hurt anything with the gain?

R
 
I agree that putting a BNC on the case is a good move. Then you can try different antennas. At 88.1 MHz, connector loss will be negligible compared to the advantage of learning by testing.

Most brands of RG58 have around 4db per 100 ft loss. If you can find an antenna type, or location, that's advantaged by 2-3 db or more at the end of 25 ft or less, it's a winner.
 
that's rather my theory - can't hurt, I can always go back to where I am right now, which I can live with...

So, I guess now the question is, what was bothering me at the beginning - center conductor of the BNC, to where the antenna is, and the other - shield - connected to ground? Since the pads for the mounting bolts to the case are rather healthy, can I attach it there?

Just don't want to blow out the output, then I'd really have to start over!

Thanks,

R
 
One more thing, RG58 is meant for a 50 ohm match, I suspect the transmitter was designed more for a 75 ohm match since a lot of those store bought FM antennas are 75 ohm. The mismatch will add more loss. The 25 ohm mismatch would be significant. Does the kit list what the antenna impedance should be?

A 25 ohm mismatch equates to a 2:1 VSWR, half power if my memory serves me right.
 
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i would think that 50 ohm transmission line would be as good as 75 ohm, but it depends on what type antenna you use if you will need a transformer. A vertical will be about 35 ohms or so depending on how far you are from its resonant frequency so no transformer would be needed at such low power. If you use a folded dipole the impedance will be a little higher, if you use commercial antenna with a 300 ohm connector (two screws for twin lead) then you will want to use a transformer at the antenna to match the coax. The RF gain control i see probably wont do any damage as long as your signal sounds clear, if you notice it is more distorted then cut it back to where its clean and build a small linear amplifier.By the way, antenna gain will improve the signal more than a small amp, it takes 10 times the power to double the signal strength at the receiver.
 
 

You make the antenna to match the transmitter, it does not matter what sort of antenna you have. Baluns have too much loss and would wipe out the transmit signal. If you use a folded dipole, or a folded sheet of aluminum foil, you set its impedance to match the source.
 
With milliwatts of power, you can't really afford too much cable loss.
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Certainly. Therefore, you should only be prepared to tolerate cable loss if you can replace it with antenna gain.
A 25 ohm mismatch equates to a 2:1 VSWR, half power if my memory serves me right.
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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..
 
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