broadband matching network
I've built a UHF broadband amp 470-802 Mhz for video transmission.
The PA is a MMG3003NT1 good for P1 24dBm.
I can get 20dBm out with good quality color video through a 1/4Wave monopole cut to 4.3". About 300 feet range.
I would like to get more power out (23 dBm) but I suspect the antenna mismatch is causing drastic reflections.
The PA is connected to the antenna through a 12pf cap.
Any help in designing a network would be highly appreciated.
Joe
gammablok@shaw.ca
Hi,
Place a isolator between PA output and antenna. You can use some circulator.
Do you have to match the whole freq range 470-802 MHz to the monopole antenna?
Hello
I think the common and direct way:
measure antenna s11 first
measure amplifier s22
design matching circuit based on s-parms measured using simulator
Circulator based solution is good time to time, but power transfer efficiency is not optimal in such case
Circulator might be very good idea, but is also expencive.
Matching network proposal by Grig is very good idea, and i think you should try it first.
Alternative solution may be ferrite insulator, it is less expencive than circulator, but more than matching network. With it you will have power loss around 0.5 dB, so design PA for max power you are able to do.
Thanks for the replies.
The circulator or isolator look good except for the size.
The smallest I've found for a drop-in is 0.75" dia.
My milled enclosure is 3x2x1" so there is not much space left.
0.5 db loss is not going to be a problem. I'm more concerned about the refections shooting through the power supply putting noise on the video.
J
Is not simple to use S Parameters to perform broadband matching for the whole band.You could try instead a series of L-C networks if you know the iput impedancnce of the antenna.
If I were you, I would have simulated (or tested) the input impedance of the antenna at the center of the band and then, assuming that your amplifier is matched at the output for the whole band, I would have used a series of 1/4 wave transmission lines (also coax).
23 dBm is not a strong power, so it could not be citical
By
Andrea
Hi Andrea,
Thanks for the ideas.
I am using a 4.3" monopole whip that resonates around 636 Mhz. The enclosure is not long enough to provide a good ground plane but it seems to transmit reasonably well. I'm getting about a 300' range with 21 dBm power out measured on the spec an.
I have not figured out how to measure the impedance of the whip yet and its time I learned. I have an old HP8754A that shows me frequency resonance points of the antenna but the impedance is another matter.
J
Added after 11 minutes:
Hi VSWR,
A late response to your question:
The unit does need to transmit on any selectable UHF channel from 14 to 69 using the one 4.3" monopole whip. The compromise will be the use of 2 or 3 different length antennas which is what I am now looking at.
I tried an isolator and it worked fairly well. The isolator was tuned to block 24 (614 to 639 Mhz). To my surprize it was quite flat from around 575 to 650 Mhz.
There was considerable insertion loss however, about 2 dB compared to the usual 0.3 - 0.5 dB. This may be telling me something about the PA behaviour.
J