Hi All,
This idea does not work for wide-band frequency changes as
claimed:
> PART 2 K7GCO:
> >Mike: This is a very good report. The L has been a very good antenna
> >even back in the 30's. I started a trend of making it longer than a
> >1/4 wave and adding a variable BC variable in series to tune out the
> >XL from being too long. It also raises the Rr closer to 50 ohms or
> >even above for 70 ohms coax. A COUPLE OF FOLKS RECOMMENDED THIS,
> >SOUNDS LIKE A GOOD POINT BUT== <SNIP> The vacuum variable is nice
> >but is a problem to adjust from the shack. The 1000 pf BC variable
> >does its thing in less than 180 degress across the whole 160 m band.
> ><SNIP AGAIN> The idea is to get the best average SWR curve from 3.5-4
> >MHz or 1.8-2 MHz or the range of frequency you will use. This
> >procedures should be obvious but there are still some that can't
> >grasp the idea.
Using an inverted L for 80 meters, with the L cut for band center at
160 ohms resistance, and fed through 17 meters of RG-71 (93
ohm, 1/4 wl @ 3.7MHz) coax, and adjusting the variable for lowest
SWR in the shack we get the following with 1500 watts:
3.5 MHz
Antenna Z = 97.34 +j666
Capacitor voltage = 3700 v peak (big receiving variable!!!)
Shack impedance = 92.6 j0
SWR = 1.85
Cable loss = 58 watts (over cable rating)
4.0 MHz
Antenna Z = 457 j1611
Capacitor voltage = 4150 v peak
Shack impedance = 19.3 j0
SWR = 2.5:1
Cable loss 65 watts
The SWR varies from 1:1 at band center to 2:1 at the low end and
2.5:1 at the high end. It really isn't any better than a regular
Inverted L with 50 ohm feeder.
It is always a good idea to model or actually build things before
deciding they work. This is the best case loss, with the reactance
cancelling capacitor at the antenna. Things get much worse if the
capacitor is moved inside the house!!!
73, Tom W8JI
W8JI@contesting.com
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