So I completed the modulation etc etc and power chain for my (conventional) FM transmitter. And I see ~ 1.4w being fed to a dummy 50 ohm load.
Now I need to feed this to an efficient/ effective antenna.
Ideally I would like to use a vertical whip. But end-fed wires are - supposedly- simple. I could go with a dipole, or even a folded dipole.
My questions is - for the best possible efficiency/ ease of implementation,
(1) which should I select
(2) how do I interface the rf amp output to this antenna. Meaning what network/ circuit/ baluns/ etc etc should I design and use for best results.
I have tried researching this and implementing a few quickie options, but clearly this is a 'black-art', and i need help & guidance !!
thanks guys
Again, follow my advice to read ARRL Antenna Book. You will find there also suitable matching devices to interface your 50 Ohm output to antenna input.
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To add one advice: transmiting a high power FM requires a license in civilized nations. Without it you may face a legal punishment with quite bad result.
If someone complains, you will be for the high jump my friend.Oh great. Now setting aside the legal issues please ?
Not simple at all. Too many variables and unknowns.Now, what I read is that the antenna impedance will appear down in the shack as a completely different complex impedance which is dependent on the coax length, amongst other things. Not a simple problem.
Yes indeed.This further leads me to think that I would need some sort of fancy ATU along with an swr-meter to actually realise a 'good' transfer of power. i.e. swr < 2:1
It only matters if you actually want to radiate some of that 1.4 watts.But surely that's too complicated ! Isn't there a simpler solution ? Or am I missing something ?
The standard solution for all antenna matching problems is a variable CLC pi matching network ("matchbox") combined with a VSWR meter.
I understand you already know that this stuff is discussed in detail in the ARRL handbook and other amateur radio publications.
If someone complains, you will be for the high jump my friend.
Not simple at all. Too many variables and unknowns.
Yes indeed.
It only matters if you actually want to radiate some of that 1.4 watts.
This would refer to an ideal voltage source with zero ohms output impedance, hard to believe for a RF amplifier. You can make amplifier with relative low output impedance (a few ohms) up to several 100 MHz, but the effort is much higher than a for simple output stage with real impedance.But here's the thing - my RF amp -and many homebrews like it- are designed to deliver a power level based on the impedance "presented to it". From the well publicised equation P = (Vc- Ve)^2 / 2.R
Nevertheless, maximum power transfer into a load will always occur when the source and load impedances are conjugate and impedance matched.The concept of "output impedance" is hard to understand or fit into this context.
Yes it will.Next issue - a basic SWR meter is designed for a nominal source & load of 50 ohm. There are several elements in a typical swr meter which are based on this. HOWEVER, if my source loading (NOT source impedance!) is based on the above equation, and my final load (antenna + coax) is a totally unknown impedance which we have agreed to above, then what is this swr meter measuring ?!? Will the swr meter give me a valid useful reading despite this ?
And I have found the ARRL Antenna books as well as the ARRL Handbooks as confusing today as they were 35 years ago, so please do not refer me to these again. I do not subscribe to any pubs. I need YOUR advice and experience for my specific questions.
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