Topband: 160 m inverted L

[Grant Saviers]

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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