RadioBanter - 50Mhz Antenna Conundrum

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50Mhz Antenna Conundrum

"charlie" wrote in message
...
Interestingly, I am able to put out a signal on 6m using a fan
dipole designed to work from 20 through 10m. Today I got a report of
5-7 over a 1600 km path. The antenna design can be seen at
http://www.radiowymsey.org/FanDipole/FanDipole.gif .

As far as I can see none of the dipoles resonate on 6m - the 40m
dipole would if it was straight dipole with no loading coils. I
guess one of the dipoles has an odd multiple near enough.

The fan dipole goes into a 50 MHz LPF and then the ICOM 703 ATU. I
wonder if anyone has an insight into how this antenna is working on 6m.

I would not be surprised if its also works on 2 meters as well.
I would think a fan dipole would operate similar to a discone.
But the explanation of how it works is simple : its magic. (fm to some)

50Mhz Antenna Conundrum

"charlie" wrote in message
...
Interestingly, I am able to put out a signal on 6m using a fan
dipole designed to work from 20 through 10m. Today I got a report of
5-7 over a 1600 km path. The antenna design can be seen at
http://www.radiowymsey.org/FanDipole/FanDipole.gif .

As far as I can see none of the dipoles resonate on 6m - the 40m
dipole would if it was straight dipole with no loading coils. I
guess one of the dipoles has an odd multiple near enough.

The fan dipole goes into a 50 MHz LPF and then the ICOM 703 ATU. I
wonder if anyone has an insight into how this antenna is working on 6m.

Charlie.

When 6 meters is hot almost anything will work. I had put together a 6
meter to 20 meter transverter running about 10 watts. Hooked it to a
trapped dipole ( 40 to 10 meters) and the first contact I made was from
North Carolina to Canada.

To see how well the antenna is working you need something to compair it to.
Say a 6 meter dipole .

50Mhz Antenna Conundrum

Interestingly, I am able to put out a signal on 6m using a fan
dipole designed to work from 20 through 10m. Today I got a report of
5-7 over a 1600 km path. The antenna design can be seen at
http://www.radiowymsey.org/FanDipole/FanDipole.gif .

As far as I can see none of the dipoles resonate on 6m - the 40m
dipole would if it was straight dipole with no loading coils. I
guess one of the dipoles has an odd multiple near enough.

Resonance of the antenna itself has little (some would say almost
nothing) with the antenna's ability to radiate. Resonant or not, if
you can get power into the radiator, it'll radiate!

Using a resonant antenna is convenient, in many situations, because it
can provide a convenient feedpoint inpedance (no reactance, and a
modest resistive impedance) which is easy to match to the feedline.

However, it's not necessary. You can radiate very effectively with a
non-resonant antenna, if you can match it to the transmitter's needs
in a way which doesn't introduce high losses. The matching can be
performed either at the transmitter (as you're doing it) or at the
antenna feedpoint. The latter is usually more efficient, but is often
less flexible.

Using non-resonant antennas, with a coax feedline, and matching at the
transmitter with an ATU, is an approach which tends to have a bad
reputation among modern hams, because of the high losses which can
result. Low-impedance coax feedlines carry relatively high currents,
and (in high-SWR situations) the I^2*R losses can add up fast and eat
most of your power. Add in the internal losses in an ATU when it's
tuned to match a difficult load, and you can end up with a pretty
weak-sounding signal.

The same non-resonant antenna, fed using a balanced ATU using
high-impedance balanced feedline, or matched at the feedpoint, could
be a lot more efficient.

My guess is that either your 17M or 20M segment is close enough to a
3/2-wavelength doublet on 6M (as you guess) so that its feedpoint
impedance isn't too terribly reactive. Referenced to a 50-ohm feed,
it might present a feedpoint impedance between 5:1 and 10:1... high
enough that you'll have some significant excess loss on the feedline,
but not enough to cripple your operation.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

50Mhz Antenna Conundrum

On Sun, 03 Jun 2007 01:12:47 +0100, charlie
wrote:

Interestingly, I am able to put out a signal on 6m using a fan
dipole designed to work from 20 through 10m. Today I got a report of
5-7 over a 1600 km path. The antenna design can be seen at
http://www.radiowymsey.org/FanDipole/FanDipole.gif .

As far as I can see none of the dipoles resonate on 6m - the 40m
dipole would if it was straight dipole with no loading coils. I
guess one of the dipoles has an odd multiple near enough.

The fan dipole goes into a 50 MHz LPF and then the ICOM 703 ATU. I
wonder if anyone has an insight into how this antenna is working on 6m.

Charlie.

I can tune my 20 meter dipole to the low end of 6 (that is, 1:1 swr)
using my mfj 986 tuner with the antenna capacitor set at about 4.75.
I'm thinking of just putting up a plain 6 meter dipole, for
comparison.

bob
k5qwg

50Mhz Antenna Conundrum

Ralph Mowery wrote:
SNIP

When 6 meters is hot almost anything will work. I had put together a 6
meter to 20 meter transverter running about 10 watts. Hooked it to a
trapped dipole ( 40 to 10 meters) and the first contact I made was from
North Carolina to Canada.

To see how well the antenna is working you need something to compair it to.
Say a 6 meter dipole .

Ralph,

Looks like I'll be up in the loft again next weekend! I had a good
weekend giving away points in the UK 6m Group contest, it's a very
interesting band.

Cheers,

Charlie.

--
M0WYM
www.radiowymsey.org

50Mhz Antenna Conundrum

Hal Rosser wrote:

I would not be surprised if its also works on 2 meters as well.
I would think a fan dipole would operate similar to a discone.
But the explanation of how it works is simple : its magic. (fm to some)

Hal,

A 2 metre rig is on my shopping list :) Not sure about the magic as
my contacts were all SSB :)

Charlie.

--
M0WYM
www.radiowymsey.org

50Mhz Antenna Conundrum

Dave Platt wrote:
SNIP
My guess is that either your 17M or 20M segment is close enough to a
3/2-wavelength doublet on 6M (as you guess) so that its feedpoint
impedance isn't too terribly reactive. Referenced to a 50-ohm feed,
it might present a feedpoint impedance between 5:1 and 10:1... high
enough that you'll have some significant excess loss on the feedline,
but not enough to cripple your operation.

Dave,

Thanks for your detailed reply, I certainly need to keep down my
losses with only 10 watts and the indoor antenna.

It's probably the 17m segment because it looks as if the 17 comes
out at around 54MHz. As suggested, next weekend I will put up a 6 m
dipole and make comparisons.

Cheers!

Charlie.

--
M0WYM
www.radiowymsey.org

50Mhz Antenna Conundrum

Charlie-- That antenna has one of its dipoles
resonate at 18.090 MHz. 3 X 18.090 = Aprox 54
MHZ ! Also, dipoles tend to be broadband
devices. This is the same reason that a 40 Meter
dipole is useable on 15 meters! And, the
SWR doesnt do much more than increase loss in the
feedline (on long runs), so it should work
at least until you get an antenna with some gain
on 6! Jim NN7K

charlie wrote:
Interestingly, I am able to put out a signal on 6m using a fan
dipole designed to work from 20 through 10m. Today I got a report of
5-7 over a 1600 km path. The antenna design can be seen at
http://www.radiowymsey.org/FanDipole/FanDipole.gif .

As far as I can see none of the dipoles resonate on 6m - the 40m
dipole would if it was straight dipole with no loading coils. I
guess one of the dipoles has an odd multiple near enough.

The fan dipole goes into a 50 MHz LPF and then the ICOM 703 ATU. I
wonder if anyone has an insight into how this antenna is working on 6m.

Charlie.

50Mhz Antenna Conundrum