Agree. I use three fixed serial caps with shorting relays to tune my
160m T, 8 elevated 125' radials, across the full band in 45KHz
segments. The antenna is cut for 1820 and fed with a 50::25 TLT.
The voltages across each cap (3 all the same value) is well below 400v
at QRO so I used surplus 1Kv mica transmit caps from Ukraine. Small
relays with 5Kv coil isolation short each cap. SPST 12a DIP relays with
very short duplex leads. Never any hot switching so they are ok +4 years.
Interestingly the series cap values are all short, one=2000, two=1000, &
three=667pf exactly move the needed for ~45KHz frequency, but other
combinations might be necessary with other T's. Use EZNEC! I use a
rotary switch with diode steering to select relay combos for each
segment over a four wire cable. I did shunt each cap with 100k ohms 2w
carbon resistor to insure static charge was shared equally.
Cost about 10% of a vacuum variable and faster to move freq. Not my
idea, thanks to somebody on TB who suggested this - a winner.
Grant KZ1W
On 11/8/2016 15:12 PM, Herbert Schoenbohm wrote:
Bread slicers have their issues and are not really the best solution.
Using a fixed high current mica G2 broadcast capacitor of a higher
value than you need, and making it variable with a series inductor is
the way to go. This is what broadcast stations do in their ATU's. I
haven't ever seen a bread slicer in a radio station ATU. A good high
current mica cap and a flat wound taped coil IMHO is the way to make
everything work well with no breakdowns. For a wide range of matching
consider a bridge T with fixed components and taped coils. Go to
W8JI's wonderful site for the values you need based on the impedance
presented to your feed-line.
Herb, KV4FZ
On 11/8/2016 6:55 PM, Rob Atkinson wrote:
A vacuum variable for L impedance matching is unnecessary. Vacuum
variable capacitors leak eventually. It take a long time for them to
go through their ranges and you have to have the mechanics outside if
you perform remote tuning, to sense or count turns to track when the
v.v. is nearing its maximum or minimum. It is far far easier and
faster to use an air variable that rotates freely. You only need to
keep it sheltered.
Elevated radials are fine provided they are high enough to be
decoupled from earth, which for most hams is difficult to accomplish
on 160 m.
73
Rob
K5UJ
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