Good afternoon, all.

I would like to put up a dipole antenna for 80 and 40 meters, and a
frequency near 4.5 MHz (for non-amateur use), coax fed, without the
use of traps or a tuner.

I expect to use three dipoles (one for each band), all coming together
in the middle and fed with a single feedline through a 1:1 W2AU-type
balun.

For illustration purposes, let's say that the 80-meter dipole will run
east-west, the 4.5-MHz dipole will run northeast-southwest, and the
40-meter dipole will run north-south.

Questions...

Is the standard 468/F formula likely to be anywhere near accurate, or
will the three antennas interact with one another to throw the
calculations off?

In particular, is the 4.5-MHz frequency too near the 80-meter band
such that interaction will be greater than otherwise?

If the interactions are sufficient to throw the formula off, will my
antennas end up being longer or shorter than the formula length?

Thanks.... CJ

I had a 5 band trapped dipole at 90 degrees to a 17M dipole fed with a
single coax and the math held true. I did have some tuning to do but just
trimming to my desired center frequency. So that was 2 dipoles (albeit one
was a five band) at 90 digress and it worked a treat.

--

Charlie
Ham Radio - AD5TH
www.ad5th.com
Live Blues Music
www.492acousticblues.com




"C. J. Clegg" wrote in message
...

Good afternoon, all.

I would like to put up a dipole antenna for 80 and 40 meters, and a
frequency near 4.5 MHz (for non-amateur use), coax fed, without the
use of traps or a tuner.

I expect to use three dipoles (one for each band), all coming together
in the middle and fed with a single feedline through a 1:1 W2AU-type
balun.

For illustration purposes, let's say that the 80-meter dipole will run
east-west, the 4.5-MHz dipole will run northeast-southwest, and the
40-meter dipole will run north-south.

Questions...

Is the standard 468/F formula likely to be anywhere near accurate, or
will the three antennas interact with one another to throw the
calculations off?

In particular, is the 4.5-MHz frequency too near the 80-meter band
such that interaction will be greater than otherwise?

If the interactions are sufficient to throw the formula off, will my
antennas end up being longer or shorter than the formula length?

Thanks.... CJ

On Sat, 21 May 2005 16:32:14 -0400, C. J. Clegg
wrote:


Good afternoon, all.

I would like to put up a dipole antenna for 80 and 40 meters, and a
frequency near 4.5 MHz (for non-amateur use), coax fed, without the
use of traps or a tuner.

I expect to use three dipoles (one for each band), all coming together
in the middle and fed with a single feedline through a 1:1 W2AU-type
balun.

For illustration purposes, let's say that the 80-meter dipole will run
east-west, the 4.5-MHz dipole will run northeast-southwest, and the
40-meter dipole will run north-south.

Questions...

Is the standard 468/F formula likely to be anywhere near accurate, or
will the three antennas interact with one another to throw the
calculations off?

In particular, is the 4.5-MHz frequency too near the 80-meter band
such that interaction will be greater than otherwise?

If the interactions are sufficient to throw the formula off, will my
antennas end up being longer or shorter than the formula length?

I'd put the 80M and 4.5 MHz dipoles normal to each other with the 40
in between.

I'd prune the 80M first and then the 4.5 and then 40M.

I run an 75-80 dipole paired with a 40 spaced about 45 deg. Can't
remember whether the formula held or not. Make 'em too long and
prune.

I have extensions on the 75 that I physically connect to go from the
phone to CW bands. Don't remember any change in the 40, if it does,
it's minor.

My first suggestion would be to use a terminated folded dipole cut large
enough for 80m and the rest would just work. Have a look at Mr Cebik's
website for details.

http://www.cebik.com/wire/t2fd.html
http://www.cebik.com/wire/wbfd.html

Your idea for the design is the kind of setup that would be easy to
model in Eznec/4nec2 etc. I did this once (only) for a 10m/6m design 90
degree offset inverted V. I noted that there was a much narrower SWR
bandwidth on the 10M side. I didnt delve into why because I never built it.

Apologies for not answering your question directly though.

Cheers Bob VK2YQA

C. J. Clegg wrote:
-----

Questions...

Is the standard 468/F formula likely to be anywhere near accurate, or
will the three antennas interact with one another to throw the
calculations off?

In particular, is the 4.5-MHz frequency too near the 80-meter band
such that interaction will be greater than otherwise?

If the interactions are sufficient to throw the formula off, will my
antennas end up being longer or shorter than the formula length?

Thanks.... CJ

On Sun, 22 May 2005 10:34:19 +1000, Bob Bob
wrote:

My first suggestion would be to use a terminated folded dipole cut large
enough for 80m and the rest would just work. Have a look at Mr Cebik's
website for details.

http://www.cebik.com/wire/t2fd.html
http://www.cebik.com/wire/wbfd.html

Good morning, Bob.

Wouldn't the terminated folded dipole be too lossy at the lowest
frequencies of interest?

In any case it looks like a terminated folded dipole would cost at
least a full "S" unit over a dipole, at any frequency (and much more
than that down by the low end of the design range).

However, looking through L.B.'s website gave me another idea ... what
about a triband full wave loop similar to what is shown in the
attached JPEG? There would be a single wire on the "fed" side of the
loop, tapped off at points along its length to create anchor points
for the other three sides of a loop for each frequency.

Have any of you tried anything like that, and how did it work?

CJ

C. J. Clegg wrote:
However, looking through L.B.'s website gave me another idea ... what
about a triband full wave loop similar to what is shown in the
attached JPEG?

Have any of you tried anything like that, and how did it work?

Unlike dipoles, there would be heavy interaction between the
loops on 40m. The feedpoint impedance of an 80m dipole is
very high on 40m and most of the power goes into the 40m
dipole. The feedpoint impedance of an 80m loop is low on
40m so the power is shared with sometimes unexpected results.
You can probably model it with the free demo version of EZNEC
available at www.eznec.com
--
73, Cecil http://www.qsl.net/w5dxp


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I'll admit that I havent played with the TFD but would agree that the
resistive termination would introduce a 3dB power loss. One has to ask
though whether that will be a problem at 80m given band noise etc.

To tell you the truth if I was trying for a 3.5, 4.5 and 7Mhz antenna
I'd probably use just two (crossed) dipoles and figure a way to
mechanically tune between the lower frequencies. Even lumped C comes to
mind.

Only HF loop I ever played with was a 40m NVIS..

Cheers Bob VK2YQA

C. J. Clegg wrote:
---
Wouldn't the terminated folded dipole be too lossy at the lowest
frequencies of interest?

Have any of you tried anything like that, and how did it work?

"C. J. Clegg" wrote in message
...

Good afternoon, all.

I would like to put up a dipole antenna for 80 and 40 meters, and a
frequency near 4.5 MHz (for non-amateur use), coax fed, without the
use of traps or a tuner.

I expect to use three dipoles (one for each band), all coming together
in the middle and fed with a single feedline through a 1:1 W2AU-type
balun.

For illustration purposes, let's say that the 80-meter dipole will run
east-west, the 4.5-MHz dipole will run northeast-southwest, and the
40-meter dipole will run north-south.

Questions...

Is the standard 468/F formula likely to be anywhere near accurate, or
will the three antennas interact with one another to throw the
calculations off?

In particular, is the 4.5-MHz frequency too near the 80-meter band
such that interaction will be greater than otherwise?

If the interactions are sufficient to throw the formula off, will my
antennas end up being longer or shorter than the formula length?

Thanks.... CJ


It should work just fine. I have an 18 MHz dipole, which also serves as the
support, and a 40 m inverted V hanging below that, fed through a current
balun. There will be interaction, so it would be a good idea to model it in
EZNEC first. You will want to trim the longest antennas first. Cut them a
little long to start with, and when you go to shorten one, don't cut off all
the extra wire, in case you have to go back. If the 4.5 is for receiving
only, it may not be worth the bother.

It took a lot of going back and forth, but I have an EZNEC simulation of a
75/80 m antenna that has an swr below 2.0 from 3.5 to 4.0. If you do put up
the 4.5 antenna, you can probably use that to extend the bandwidth of the 80
m antenna on the high side.

Tam/WB2TT

"Tam/WB2TT" wrote in message
...

"C. J. Clegg" wrote in message
...

Good afternoon, all.

I would like to put up a dipole antenna for 80 and 40 meters, and a
frequency near 4.5 MHz (for non-amateur use), coax fed, without the
use of traps or a tuner.

I expect to use three dipoles (one for each band), all coming together
in the middle and fed with a single feedline through a 1:1 W2AU-type
balun.

For illustration purposes, let's say that the 80-meter dipole will run
east-west, the 4.5-MHz dipole will run northeast-southwest, and the
40-meter dipole will run north-south.

Questions...

Is the standard 468/F formula likely to be anywhere near accurate, or
will the three antennas interact with one another to throw the
calculations off?

In particular, is the 4.5-MHz frequency too near the 80-meter band
such that interaction will be greater than otherwise?

If the interactions are sufficient to throw the formula off, will my
antennas end up being longer or shorter than the formula length?

Thanks.... CJ


It should work just fine. I have an 18 MHz dipole, which also serves as
the support, and a 40 m inverted V hanging below that, fed through a
current balun. There will be interaction, so it would be a good idea to
model it in EZNEC first. You will want to trim the longest antennas first.
Cut them a little long to start with, and when you go to shorten one,
don't cut off all the extra wire, in case you have to go back. If the 4.5
is for receiving only, it may not be worth the bother.

It took a lot of going back and forth, but I have an EZNEC simulation of a
75/80 m antenna that has an swr below 2.0 from 3.5 to 4.0. If you do put
up the 4.5 antenna, you can probably use that to extend the bandwidth of
the 80 m antenna on the high side.

Tam/WB2TT

CJ,
I ran an EZNEC simulation on a pair of crossed dipoles at 3.7 and 4.5 MHz,
with a 7.2 MHz inverted V below the 3.7. Not good. The 7.2 works OK, but the
4.5 really messes up the 3.7. Among other things the impedance at 3.7 and
4.5 MHz is about 115 Ohms. I ran the simulation at 50 feet above average
ground. If the 4.5 is for receive only, I would forget about it, and take
the 10:1 SWR that the 80 meter dipole gives you.

Tam/WB2TT

On Sun, 22 May 2005 17:41:22 -0400, "Tam/WB2TT"
wrote:

I ran an EZNEC simulation on a pair of crossed dipoles at 3.7 and 4.5 MHz,
with a 7.2 MHz inverted V below the 3.7. Not good. The 7.2 works OK, but the
4.5 really messes up the 3.7. Among other things the impedance at 3.7 and
4.5 MHz is about 115 Ohms. I ran the simulation at 50 feet above average
ground. If the 4.5 is for receive only, I would forget about it, and take
the 10:1 SWR that the 80 meter dipole gives you.

Good evening, Tam.

Thanks for running the simulation for me. Your results don't surprise
me much.

(I'm going to have to get that EZNEC program and play with it for a
while ... thanks to all for mentioning it.)

The 4.5-MHz antenna will be used for transmitting as well as
receiving, and so I need to get the SWR down to something reasonable.

I guess there's no reason why I can't just put up a separate dipole
for 4.5 MHz and the crossed dipoles, fed with a single coax, for 80
and 40.

I could also (and I might...) just put up an 80-meter dipole fed with
ladder line and use a tuner, though I was trying to avoid the tuner.

Thanks (to you and to all) for saving me a bunch of work. :-)

CJ