I am looking at this manual which I think i'll give a shot. I have plenty of heat sinks laying around from old computers, etc.
https://www.electro-tech-online.com/custompdfs/2009/12/dl_30w_hf-uhf-2.pdf
I see how you connect the resister to the BNC connector. I assume the transmitter then connects to the BNC connector? So where does the antenna come in? It seems like a one ended doummy load. Does the signal actually transmit from the dummy load without an antenna? Am I missing something?
Thanks
I believe that is the "homebrew" dummy load that I suggested you look at earlier.
All it is, frankly, is a load resistor. But to understand why it does transmit "leakage" electromagnetic energy you have to investigate and learn about AC circuits, particularly RLC (resistive, inductive, capacitive) circuits and filters.
All an antenna amounts to is a resonant band-pass filter. Antennas by their nature are capacitive, inductive, and resistive elements. When an antenna is designed to be resonant at a particular frequency, it will present a 50 ohm IMPEDANCE, well actually 68 ohms but that is an unimportant theory tidbit, which is a combination of inductive and capacitive reactance as well as resistance, to the amplifier stage of the transmitter. This is too complicated to describe in a few paragraphs. Just know that your supplied FM transmitter antenna is designed this way.
When the antenna is resonant the capacitive and inductive properties will be in balance, leaving the characteristic impedance of the feedline to essentially be the only thing the transmitter sees, which in this case is 50 ohms and purely resistive. The feedline has its own inductive, capacitive, and reactive elements as well. You can actually cut a piece of coax to a certain length and the transmitter will see 50 ohms without an antenna attached. A 50 ohm resistor at the end of a short piece of coax will take the place of the 50 ohm resonant impedance of the antenna and present the proper impedance to the transmitter. However, a resistor is not a reactive element and thus will not, in a perfect world, radiate an electromagnetic field, which is the property of inductors and capacitors.
HOWEVER, a resistor is never a perfectly resistive, non-reactive component. The leads of a resistor are straight wires and will radiate as an inductor. The leads are often within proximity to each other and thus form capacitive plates. In some resistors there is a shunt impedance in the resistive element itself and small series capacitance within. The higher the frequency, the more these reactive conditions within the resistor become evident.
It is best to choose a resistor for a dummy load that mitigates much of the reactance that other resistors can't. For one, because you can't predict that the impedance will remain 50 ohms without doing calculations. The Thin film resistor in the project is a good example of one that will have minimal reactive properties. But no resistor is perfect in the real world so a 100 MHz even this resistor will radiate a certain amount.
This is why dummy loads still manage to radiate. Between the connectors, the leads, and the internal reactance in the resistor, you will have leakage radiation. In the case of 5 watts terminated into it, you will probably have plenty of radiation to be picked up by a FM reciever within the confines of your property, on an unused frequency where no interference can occur.
This is why I suggested building a dummy load from the start.
I hope the discussion about AC theory wasn't too confusing. I may have missed a few things in trying to simplify the concept but you can learn more on the internet.