Topband: Antenna Question, Sorry

[Terry Conboy]

At 07:02 AM 2005-08-10, Kevin Nathan wrote:
>looking at the antenna situation here I have the room to put in a 
>ground mounted inverted L antenna of approximately 150 to 170 feet 
>in length with perhaps 60 or 70 feet vertical.  At present, I have 
>300 PF of capacitance in the form of a couple of fixed caps rated at 
>a working voltage of 2500.  I know if I put these or even more 
>capacitance in series, I can tune the SWR to 1 to 1 on 160.
>
>My question though regards possibly using this antenna on 80 and 40 
>for DX work.  I could simply feed it with coax, not put the caps in, 
>and just use my tuner to attempt to deal with the SWR which might be 
>involved.  If possible, can some of you share opinions with me 
>regarding how you feel this antenna would play on 160 and also on 80 and 40?

Kevin,

This antenna should perform well for DX on 160m.  With 65 foot 
vertical and 95 feet horizontal wire, the feed impedance is about 30 
ohms plus ground loss R in series with about 189 ohms of inductive 
reactance (which would take about 460 pF to tune out).

Without the series cap, the SWR will be about 20:1 and 100 feet of 
RG-213 would have 1.7 dB loss (vs. 0.25 dB matched) at 1.83 MHz.  (10 
ohm ground loss resistance assumed.)

However, 80m is not so favorable in two respects.  The horizontal 
wire has the current maxima and the the vertical wire has the high 
voltage point, so most of the radiation is from the horizontal wire 
and the pattern is similar to a dipole at that height.  Since it is 
about 1/4 wavelength high, the main lobe is straight up.  Fine for 
short to medium distance contacts.

In addition, the feed impedance is fairly high on 80m, with SWR of 
about 94:1 at 3.5 MHz and 30:1 at 4 MHz.  The 100 feet of RG-213 
would have 6.7 dB loss at 3.5 MHz and 3.5 dB loss at 4 MHz.  (With 
170 feet total wire length, the match would improve slightly.)

Using the 80m trap would probably be a good idea, since it would 
essentially give you a 1/4 wavelength vertical on 80m and better DX 
performance.  It would also make the feed impedance much closer to 50 
ohms for lower line loss.  The series cap for 160m could be left in 
place on both bands with a suitable placement of the 80m trap.

73, Terry N6RY