Originally Posted by OTAzak
Are there any opinions as to whether Jointennas should be used before or after signal amplifiers? I have always used them after amplification....
If I recall correctly jointennas are power passive.
Originally Posted by rabbit73
Maybe one way, but not the other:
I just popped the cases open on an 0579 channel 9 VHF Jointenna and an 0585-1 UHF Jointenna pre-tuned to channel 28.
That schematic above is for a VHF Jointenna, which does not pass power through its single antenna port. The UHF 0585-X Jointennas DO pass power to all ports. I just measured zero resistance from TV Set to All Channels, and 0.5 ohms from TV Set to Single Channel. I am not able to estimate their current carrying capability of the Single Channel circuit, as those current bypass inductors are wound with awfully tiny wire, but they surely can carry whatever amount of current a single preamplifier draws. FWIW, the TV to All Channels path can carry enough current to light a Christmas tree,
In fact, the drawing is either a Low Band Jointenna or a very old one. The channel 9 High Band Jointenna I am looking at right now was likely made in this century, and it has tiny trimmer inductors in series with each of the shunt capacitors. I have not yet experimented with them to see what their tuning range and effects is.
About a decade ago, I opened and examined some low band Jointennas whose packages I had to wipe the dust off to read, and they did not have similar, trimmer inductors. I have no way of knowing whether those trimmers are a modern addition or an inherent design difference between the low band and high band units.
The two big inductors on the VHF Jointennas are wound around the same threaded non inductive core, meaning they function as an air core transformer. Curiously, that core is threaded and slotted and can be turned, but turning it accomplishes nothing electrically, so it was probably used just because it was a readily available item that neatly supports those heavy gauge four turn coils.
On the UHF Jointenna, the similarly circuited inductor coils that also interact to function as a transformer are free standing and appear to have been hand optimized by poking at them. There are also multiturn inductor screw slugs that move the pass band and reject notch through the entire designated ranges of channels 14-30 (0585-1), 31-49 (0585-2), and presumably 50-69 (0585-3), but I don't have an 0585-3 available to test. FWIW, when I adjust those inductor slugs to change the UHF Jointennas from one channel to another, I see no effect of a metal, inductive screwdriver of their performance. Similarly, I could tune the slugs on the old, Tru-Spec BPFs with an ordinary, 1/8" magnetic screwdriver as well.