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mast mounted bi-amp

Jun 10th 2013, 23:17


Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I am having my antenna a great distance from my ham rig as I have posted many times on this site (kite in tree).
I will be mast mounting the bi-amp some distance from my ham rig and using 300ohm #22 gauge open wire feeder for stealth (white wire appears as kite string!)
I need to find the values of chokes to power the circuit and estimate it will draw about 4 or 5 amps.
Just by common sense I would pick the highest inductance depending on the lowest frequency hopefully 10-80 mhz and the gauge of wire wound would be approximately 16 gauge, weight is critical.
How is this going to effect the impedance of the open wire? which seems to be almost impossible to find sometimes (tap).
I'm afraid the chokes will simply raise the impedance too high or act as load and drop to short.
Jun 11th 2013, 12:38


Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

This is actually a well known and difficult problem to solve in ham radio--better known as "why can't someone design a high current plate choke that works on all bands that won't blow up?"

Most designs don't try for the ideal--a choke that works well over a broadband frequency range. Instead, the usual technique is to get out the test gear, and tweak the coil such that the weak spots end up on frequencies that will never be used. If you have suitable test equipment, you could measure your antenna system to figure out where the choke needs to work really well.

Zack Lau W1VT
ARRL Senior Lab Engineer

Jun 18th 2013, 20:27


Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Oh I know I should have thought of some of the collector to emitter circuits I built (following instructions!) using so many turns and studied the plate b+ circuits in ham school many years ago, but had to post this to remember!
Now I have another issue...I'm using a coax balun at the ham rig and it shows a shunt, I am hesitant to use capacitor blocking.

I think about if I just had the bi-amp picking up the rf much like a high impedance test probe, I could transmit enough energy at the shack to power the circuit, of course I would have to run some power to charge a battery defeating the purpose of running high current dc in the first place.
Actually I don't really need extra power for transmit even if the loss is so great that it becomes almost a part 15 device! it's more about the receive amp, although with twin lead, the reasons for using coax and cutting down on harmonics makes using antenna mounted amp a must.

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