Those little Mica's need to handle a lot of current so unless you want
them to explode with a bang I would put enough small values in parallel
to come up with the needed series capacitance. The best solution is a
vacuum variable but that is some money if you get one with flanges for
mounting. Before I got mine here I used a calculated length of LMR-400
made into a coax capacitor and coiled up at the base of the tower The
capacity per foot varies but is published and can work. But I like the
flexibility of a vac in the tuning box. Bread slicers are my last choice.
The other solution is to get a nice G2 mica at a higher value than you
need (any value) and put a tapped coil in series with it to make it
variable. Preferably a flat wound coil and tap like are used in
broadcast ATU's.
Herb Schoenbohm, KV4FZ
On 8/2/2015 8:30 PM, Grant Saviers wrote:
I found NOS Soviet block screw mount micas on ebay that looked like
they would handle QRO and have three pairs in series to cover the band
with shorting relays - all shorted = 1820 then each step (2 short, 1
short, all open) adds 50KHz. 85' to top, 2x 30' hat and 8 10'
elevated 125' radials. So far no problems. Relays are PCB 8a, 5KV
coil isolation, they only see 350v or so and never hot switch. Ok so
far, up 2 years but I think the top is a little too close to the
tree. The tree guy will fix that next visit.
Grant KZ1W
On 8/2/2015 17:10 PM, Steve Flood wrote:
My top-loaded vertical needs a series 260pF capacitor at the
feedpoint to
match the 53 + j335 impedance (1.825 MHz) to my coax feed.
I have a few different air-variables I could use, but I was wondering
what
sort of fixed capacitors would be suitable in this application.
Max power is 1500w. I have doorknob caps but they don't have current
ratings marked on them.
Thanks,
Steve KK7UV
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