Good comments from everyone. Thanks! One might think that this was a
loaded question - it was! I spent the last week or so taking an
alternative approach. See below...
Here is what I came up with. First having the luxury of another tower
being available for other antenna projects, I built a rotary dipole at 50
ft. tuned for 7.2 MHz (very much like a Cushcraft design with loading
coils, capacity hat, etc). While experimenting with this antenna, I
coupled it into the other 40 M antenna (2 el. at 100 ft.) and had the
ability to "stack" them with appropriate impedance matching
(upper/lower/both switch box), I discovered that when the dipole was tied
into the 2 el. tuned for CW ("both antenna" switch position), the SWR got a
whole lot better in the phone band and the amplifier worked much better
(full power without it tripping the safety's). Some on the air tests
revealed that indeed the gain of the 2 element was close to 1-S Unit over
the dipole. Good - that works!
I took it a bit further and added a director to the dipole (same
construction technique as the DE) and interlaced it with an existing 4
element 10 M yagi. Lo and behold, not only did the SWR remain good on SSB,
but also saw close to 2-dB of gain over the individual antennas in the
phone portion of the band! Horizontal stacking here! Upper antenna at 100
ft. and then diagonally stacked to the new 2 element at 50 ft. or probably
close to 1-1/2 wavelength spacing.
Call me crazy, but then spent a few bucks on some relays and built up a
control box that allows me to go from CW to SSB with the flip of a switch;
select the new 2 element SSB antenna independently, run a combination of 2
over 3 (have a third 40 M yagi too (3 element Force 12) that can be stacked
with the upper 2 element (100 ft.)) on CW, and with the new 2 element
added, I get 7 elements on line. The control box has 3 toggle
switches, one switch gives me the Hi 2 element, Low 3 element at Europe and
both in phase (center off position); another switch selects the 2 element
SSB and a third switch changes the whole setup from CW to SSB which adds
the 2 element on SSB back into the circuit. This antenna is added just
ahead of the upper/lower/both switching circuit without any phase matching
considerations, so it improves the SWR on both of the CW set antennas. I
have noticed that when I use the CW antennas in the phone portion of the
band (2 high or 3 element low at EU), the signal level does drop, probably
due to the mismatch on the line and other factors, but as soon as I switch
the new SSB antenna in line, the signals jump back up.
So going beyond the "add more feed line" or "change the original 2 element
design to resonate between the CW and Mid settings to cover the entire
band", I have opted to play with the feed line plus pick up a couple more
dB with horizontal stacking! I find now when all 3 antennas are facing EU
or SW, gain appears to be increased. In other directions, I don't see much
improvement due to horizontal stacking as one antenna is behind the other
and I guess the gain falls off due to a negative phase shift. The antennas
are side by side when facing Europe or the SW direction. The antenna on the
50 ft. tower is also rotatable.
Of course, we'll see if this really works in the upcoming CQ WW SSB test!
Just love this antenna experimentation stuff!
Regards,
Mark, K1RX
Chuck Counselman <ccc@space.mit.edu> on 10/02/2002 11:00:05 AM
To: Mark Pride/Bedford/IBM@IBMUS
cc:
Subject: Re: [Towertalk] 40 m yagi matching
>Fellow TTer's: Problem - Have a 2 element yagi on 40 M that is currently
>tuned for the CW end of the band. In order to use it on the SSB end, past
>techniques have included....
What is the SWR at the CW end, and what is it at the 'phone end of the
band?
I don't have the same antenna, but what I did was to run Andrew
Heliax type LDF5-50 ("7/8ths-inch") all the way (65 feet) from my amp
to a fixed matching network and balun at the feedpoint of my antenna.
I run legal-limit power and routinely operate with SWR on the Heliax
equal to 6 or 7. A transmission-line calculation showed that the
peak voltage and current on the Heliax would be within its spec's,
and that the loss would be negligible.
-Chuck, W1HIS
|