"Buck" wrote in message
...
An earlier discussion brought to mind the question of the 3/2 wl
antennas such as using a 40 meter dipole to operate on 15 meters. I
just modeled one using the following stats:

1 wire, 66 feet high, 68.8286 feet long. I then pulled an SWR reading
from it from 3.5 to 30 MHz using .1 increments. The result was, of
course, two dips, one at 7.0 MHz and one at 21.1. By playing with the
alternative impedance, I found that 94 ohms matched both frequencies
at about 1.15:1.

Here are some observations about it.

at 7.0 MHz, the shows:

50 ohms -- 1.65:1
72 ohms -- 1.14:1
75 ohms -- 1.09:1
94 ohms -- 1.14:1

at 21.1 MHz, I get the following:
50 ohms -- 2.16:1
72 ohms -- 1.5:1
75 ohms -- 1.44:1
94 ohms -- 1.15:1


The above are the lowest SWR readings (which is why 21.1 instead of 21
MHz)

so, how does one make a 94 ohm match?


--
73 for now
Buck
N4PGW

You might try this, connect a 50 Ohm feedline to the antenna through an
electrical 1/4 wave section of 75 Ohm coax. That should help the SWR on 21,
without much effect on 7.

BTW, Roy mentioned bending the elements to form a V. That is what is done on
some VHF TV antennas. They work on fundamental mode on channels 2 -6 and 3/2
mode on channels 7 - 13. If you didn't bend the elements, channels 7 - 13
would not come in from the same direction as 2 - 6.

Tam/WB2TT

"Tam/WB2TT" wrote in message
...

"Buck" wrote in message
...
An earlier discussion brought to mind the question of the 3/2 wl
antennas such as using a 40 meter dipole to operate on 15 meters. I
just modeled one using the following stats:...........................


Buck,
I ran EZNEC on your antenna, and the 1/4 wave section works like a charm.
However it does not do anything to bring down the frequency of the 3/2 mode.
You need to use another TV antenna trick. On a VHF antenna, elements that
are resonant on CH5 and CH6 do no good in the 3/2 mode because they are
resonant above CH13. What is done is this: you put a CAPACITOR in the
element, like a loading coil would be. This forces you to make the element
longer to work on the fundamental. The capacitor has less effect on the 3X
frequency; So, you can actually have the 3X resonance be less than 3 times
the 1X resonance.

Here is what I got:

Length +/- 34.5 feet
1/4 Wave section of 75 Ohm coax.
100 PF capacitor in each half of the dipole, about 5% in from each end.
SWR=1.5 at 7.0 MHz (50 Ohm)
SWR=1.1 at 21.2 MHz (50 Ohm)

I took a guess at the 100 PF number, and the capacitor placement; so, you
can optimize it. Note that the further you move the capacitors from the ends
of the antenna, the more effect they will have on the fundamental.

Tam/WB2TT