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

On Tue, 14 Jun 2005 17:28:30 -0400, Buck wrote:

so, how does one make a 94 ohm match?

Hi Buck,

Dip a 94 Ohm resistor in a solution of Red Phosphorus and Potassium
Chlorate and let it dry. ;-)

73's
Richard Clark, KB7QHC

Buck wrote:

so, how does one make a 94 ohm match? [at both 7 and 21 MHz]

1. A 2:1 transformer.
2. An LC network. I think at least three components would be necessary
for matching at both frequencies, and an exact match would probably
require four.
3. A number of different schemes using transmission line stubs.
4. Don't bother. There's nothing wrong with a 2:1 SWR. Feedline loss
will reduce it further.

A couple of other notes:

I often use a 40 meter dipole for 15 meters on Field Day. I put a small
inductor at the feedpoint to bring the resonance down to the bottom end
of 15; it doesn't have much effect at 7 MHz. (If it did, I could get
resonance on both bands by alternately adjusting the antenna length and
inductor value.)

You can make a nice bidirectional pattern with some gain in two specific
directions on 15 by bending the wires toward each other by 30 degrees to
make a horizontal vee with 120 degree included angle. This only has a
minor effect on the pattern on 40 meters.

Roy Lewallen, W7EL

"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

Buck,
Why not use a tuner?
'Doc

On Wed, 15 Jun 2005 03:52:52 GMT, 'Doc wrote:

Buck,
Why not use a tuner?
'Doc

For some dumb reason, I prefer to have a 'switch and transmit'
operation. On the CHN, I have a dedicated antenna matched
specifically for the 20 meter net frequency. Right now I use the
tuner on the 40 meter net so I just switch between 40 and 20 meters.
I will eventually build a 40 meter dipole with the 20 hung under it so
I don't even have to switch when I change bands.

As a novice I used a 40 meter dipole that matched perfectly on 15
meters. I just modeled that antenna to see how it turned out on the
rig. This is how I came to see that one impedance matched both bands
perfectly. (perfectly being defined as low SWR across the desired
portion of the band.)


--
73 for now
Buck
N4PGW