Here are a couple messages I posted on eHam net. Any comments would be
appreciated.

Thanks,

Doug VE3XDB

--
MESSAGE #1:

Hi everyone,

I am thinking of building a center loaded fan dipole. One wire dipole
to cover 20 and 40 metres, and a second wire diple, connected at the
same feedpoint, to cover 30 and 17 metres. Coils would be placed at the
resonant point of the higher frequency, acting as both an RF choke and
a loading coil. My question is this. I would like to have the wire
beyond the coil as long as possible, but would still like the coil to
have enough inductance to act as an effective RF choke for the higher
band.

Other than through experimentation, how would one calculate or
determine this mix of inductance/length?

Best regards,

Doug VE3XDB

--
MESSAGE #2:

Thanks for your replies. I know the technique does work. The Alpha
Delta DX-CC puts a loading coil at the end of a 40 meter dipole, and
adds about 9 feet of wire past the coil. This creates a dipole that
resonates on 40 and 80 meters. Here is a link:

http://www.alphadeltacom.com/pg1.htm

As well, coil loaded HF antennas are described in "The Radio Amateur
Antenna Handbook" by Bill Orr W6SAI and Stuart Cowan W2LX (Published in
1993 by Radio Amateur Callbook, a Division of BPI Communications Inc.,
pages 147 to 151).

Typical dual band antennas, of the type used for 2 meter/70 cm
operation mobile are coil-loaded whips. Reg G4FGQ wrote a program
called LOADCOIL.EXE that calculates the characteristics of a loading
coil at any point on an antenna radiator of a given length.

I know the length of wire necessary to resonant the antenna at the
higher frequency, and can figure out the coil required based on the
length of wire past the coil. What I don't know is what is the maximum
wire length, and therefore minimum inductance, necessary to have the
antenna resonate on 2 bands.

Here is what I calculated, using Reg's program.

20-40 meter combination (resonant at 14.150 and 7.150 MHz)

Feedpoint to coil 16.54'
Coil 17.73 uH
Coil to end of antenna 7.46'

30-17 meter combination (resonant at 10.120 and 18.100)

Feedpoint to coil 12.93'
Coil 11.21 uH
Coil to end of antenna 5.07'

This creates an antenna that is about 75% full size on 30 and 40
meters, full size on 17 and 20 meters.

What I'm trying to determine is the maximum length of the radiator (and
minimum inducatance) necessary to achieve dual band capability.

Any insights?

Best regards,

Doug VE3XDB

you don't want to think 'loading coils' you want to think 'traps'. they are
two very different animals. loading coils are simply inductors that make
the wire look longer electrically so you don't need as much room. traps are
resonant circuits that exhibit a very large impedance at their tuned
frequency so that it cuts off part of the antenna. usually you use a trap
for multi-band dipoles, the first trap out from the center is tuned to cut
off the outside of the element at the highest frequency, and then at lower
frequencies it looks more like a small inductor so helps a bit as a loading
coil on the lower band.

wrote in message
ups.com...
Here are a couple messages I posted on eHam net. Any comments would be
appreciated.

Thanks,

Doug VE3XDB

--
MESSAGE #1:

Hi everyone,

I am thinking of building a center loaded fan dipole. One wire dipole
to cover 20 and 40 metres, and a second wire diple, connected at the
same feedpoint, to cover 30 and 17 metres. Coils would be placed at the
resonant point of the higher frequency, acting as both an RF choke and
a loading coil. My question is this. I would like to have the wire
beyond the coil as long as possible, but would still like the coil to
have enough inductance to act as an effective RF choke for the higher
band.

Other than through experimentation, how would one calculate or
determine this mix of inductance/length?

Best regards,

Doug VE3XDB

--
MESSAGE #2:

Thanks for your replies. I know the technique does work. The Alpha
Delta DX-CC puts a loading coil at the end of a 40 meter dipole, and
adds about 9 feet of wire past the coil. This creates a dipole that
resonates on 40 and 80 meters. Here is a link:

http://www.alphadeltacom.com/pg1.htm

As well, coil loaded HF antennas are described in "The Radio Amateur
Antenna Handbook" by Bill Orr W6SAI and Stuart Cowan W2LX (Published in
1993 by Radio Amateur Callbook, a Division of BPI Communications Inc.,
pages 147 to 151).

Typical dual band antennas, of the type used for 2 meter/70 cm
operation mobile are coil-loaded whips. Reg G4FGQ wrote a program
called LOADCOIL.EXE that calculates the characteristics of a loading
coil at any point on an antenna radiator of a given length.

I know the length of wire necessary to resonant the antenna at the
higher frequency, and can figure out the coil required based on the
length of wire past the coil. What I don't know is what is the maximum
wire length, and therefore minimum inductance, necessary to have the
antenna resonate on 2 bands.

Here is what I calculated, using Reg's program.

20-40 meter combination (resonant at 14.150 and 7.150 MHz)

Feedpoint to coil 16.54'
Coil 17.73 uH
Coil to end of antenna 7.46'

30-17 meter combination (resonant at 10.120 and 18.100)

Feedpoint to coil 12.93'
Coil 11.21 uH
Coil to end of antenna 5.07'

This creates an antenna that is about 75% full size on 30 and 40
meters, full size on 17 and 20 meters.

What I'm trying to determine is the maximum length of the radiator (and
minimum inducatance) necessary to achieve dual band capability.

Any insights?

Best regards,

Doug VE3XDB

I believe EZNEC will model this but any modeling software is going to
require a good deal good deal of tinkering when ported to real life
with so many lumped circuit variables.

Placing the coils at the end of an otherwise resonate section seems
like a good idea but it just is not that simple. The 40 meter
operation will essentially be a center loaded dipole.

If you choose to go this direction I would think it best to do one
dipole pair at a time. Also, study the commercial products carefully.

My preference would be four dipoles (one for each frequency) in a fan
arrangement.

de W8CCW John

On 2 Jul 2006 11:28:38 -0700, wrote:

Here are a couple messages I posted on eHam net. Any comments would be
appreciated.

Thanks,

Doug VE3XDB

--
MESSAGE #1:

Hi everyone,

I am thinking of building a center loaded fan dipole. One wire dipole
to cover 20 and 40 metres, and a second wire diple, connected at the
same feedpoint, to cover 30 and 17 metres. Coils would be placed at the
resonant point of the higher frequency, acting as both an RF choke and
a loading coil. My question is this. I would like to have the wire
beyond the coil as long as possible, but would still like the coil to
have enough inductance to act as an effective RF choke for the higher
band.

Other than through experimentation, how would one calculate or
determine this mix of inductance/length?

Best regards,

Doug VE3XDB
John Ferrell W8CCW

wrote in message
ups.com...
Here are a couple messages I posted on eHam net. Any comments would be
appreciated.

Thanks,

Doug VE3XDB

--
MESSAGE #1:

Hi everyone,

I am thinking of building a center loaded fan dipole. One wire dipole
to cover 20 and 40 metres, and a second wire diple, connected at the
same feedpoint, to cover 30 and 17 metres. Coils would be placed at the
resonant point of the higher frequency, acting as both an RF choke and
a loading coil. My question is this. I would like to have the wire
beyond the coil as long as possible, but would still like the coil to
have enough inductance to act as an effective RF choke for the higher
band.

Other than through experimentation, how would one calculate or
determine this mix of inductance/length?

Best regards,

Doug VE3XDB

Regardless of what you do, this is going to be guaranteed to have lousy
bandwidth on 40 meters (and 30, but you don't care). The whole thing might
be easier if you have 4 dipoles at a common feedpoint. I haven't tried 4,
but did have 3: 40, 17, and 12 meters. You will want to have angular
separation between the dipoles, either horizontal or vertical, or a
combination. I did not measure it, but probably had about 15 degrees of
separation. Tune by trimming the lowest frequency antenna first.

If you still want the trap dipoles, you can always buy the traps from places
like AES. They will probably come with instructions. With 2 trap dipoles at
a common feedpoint, I suspect this will be very difficult to tune.

Tam/WB2TT

My antenna for 160/80/40/30 is as follows:

160m loaded dipole shortened to 200' with 30uHy inductors so it will fit in
the lot.
80/40 parallel fan full size dipoles at right angles to 160m dipole
30m the loading coils on the 160m dipole act as RFC's or traps on 30m to
isolate the center section of the dipole as a full size dipole on 30m.
All with a common feed point through a home brew 1:1 current balun.

I was modeling the whole array before moving the loading coils in EZNEC and
noticed that I had a near perfect resonate point at about 6MHz defined by
the loading coils on the shortened 160m dipoles center loading coils. As
this is in a shortwave broadcast band it was hitting the front end of my
radio with very strong signals. As I'm a DXer I didn't like the idea that
strong out of band signals may have been degrading my in band receive
performance. The idea of moving the coils in to make a 30m dipole came to
me as a good way to fix the problem and give me a antenna on a band I had no
antenna for. To keep the length of the 160 dipole at 200' when moving the
loading coils in to make 30m dipole required increasing the inductance to
30uHy's. The SWR is a reasonable 1.2:1 and seems to work quite well. As 160
and 30m are the only bands I don't have DXCC on, I'm making good progress on
30m to getting it, not so good on 160m as I don't have an amplifier on
160m.


--
John Passaneau W3JXP
State College Pa.


wrote in message
ups.com...
Here are a couple messages I posted on eHam net. Any comments would be
appreciated.

Thanks,

Doug VE3XDB

--
MESSAGE #1:

Hi everyone,

I am thinking of building a center loaded fan dipole. One wire dipole
to cover 20 and 40 metres, and a second wire diple, connected at the
same feedpoint, to cover 30 and 17 metres. Coils would be placed at the
resonant point of the higher frequency, acting as both an RF choke and
a loading coil. My question is this. I would like to have the wire
beyond the coil as long as possible, but would still like the coil to
have enough inductance to act as an effective RF choke for the higher
band.

Other than through experimentation, how would one calculate or
determine this mix of inductance/length?

Best regards,

Doug VE3XDB

--
MESSAGE #2:

Thanks for your replies. I know the technique does work. The Alpha
Delta DX-CC puts a loading coil at the end of a 40 meter dipole, and
adds about 9 feet of wire past the coil. This creates a dipole that
resonates on 40 and 80 meters. Here is a link:

http://www.alphadeltacom.com/pg1.htm

As well, coil loaded HF antennas are described in "The Radio Amateur
Antenna Handbook" by Bill Orr W6SAI and Stuart Cowan W2LX (Published in
1993 by Radio Amateur Callbook, a Division of BPI Communications Inc.,
pages 147 to 151).

Typical dual band antennas, of the type used for 2 meter/70 cm
operation mobile are coil-loaded whips. Reg G4FGQ wrote a program
called LOADCOIL.EXE that calculates the characteristics of a loading
coil at any point on an antenna radiator of a given length.

I know the length of wire necessary to resonant the antenna at the
higher frequency, and can figure out the coil required based on the
length of wire past the coil. What I don't know is what is the maximum
wire length, and therefore minimum inductance, necessary to have the
antenna resonate on 2 bands.

Here is what I calculated, using Reg's program.

20-40 meter combination (resonant at 14.150 and 7.150 MHz)

Feedpoint to coil 16.54'
Coil 17.73 uH
Coil to end of antenna 7.46'

30-17 meter combination (resonant at 10.120 and 18.100)

Feedpoint to coil 12.93'
Coil 11.21 uH
Coil to end of antenna 5.07'

This creates an antenna that is about 75% full size on 30 and 40
meters, full size on 17 and 20 meters.

What I'm trying to determine is the maximum length of the radiator (and
minimum inducatance) necessary to achieve dual band capability.

Any insights?

Best regards,

Doug VE3XDB

On Sun, 2 Jul 2006 18:43:35 -0000, "Dave" wrote:

you don't want to think 'loading coils' you want to think 'traps'. they are
two very different animals. loading coils are simply inductors that make
the wire look longer electrically so you don't need as much room. traps are
resonant circuits that exhibit a very large impedance at their tuned
frequency so that it cuts off part of the antenna. usually you use a trap
for multi-band dipoles, the first trap out from the center is tuned to cut
off the outside of the element at the highest frequency, and then at lower
frequencies it looks more like a small inductor so helps a bit as a loading
coil on the lower band.



Actually, if you check the ARRL antenna handbook or the annual
handbook, or maybe even the web site, they have a dual band 80/40
meter dipole that uses loading coils instead of traps. One version
uses a drop wire at the end of the 40 meter band to trim without
moving the coils or adjusting the 80 meter frequency.

I haven't tried it, but I have seen other antennas designed on it's
principal, mainly vertical 1/4 wave antennas, but they worked so I see
no reason the dipole shouldn't.

Will the fan-dual-band dipole work? I suspect so. However, I highly
suspect that the parallel dipole (a fan with the elements 6-8 inches
below each other) will work better.

Good luck
Buck

--
73 for now
Buck
N4PGW

On Sun, 2 Jul 2006 18:43:35 -0000, "Dave" wrote:

you don't want to think 'loading coils' you want to think 'traps'. they are
two very different animals. loading coils are simply inductors that make
the wire look longer electrically so you don't need as much room. traps are
resonant circuits that exhibit a very large impedance at their tuned
frequency so that it cuts off part of the antenna. usually you use a trap
for multi-band dipoles, the first trap out from the center is tuned to cut
off the outside of the element at the highest frequency, and then at lower
frequencies it looks more like a small inductor so helps a bit as a loading
coil on the lower band.

Yes, but the trap (which is a parallel tuned circuit) doesn't need to
be resonant in one of the operating bands, and there are reasons for
choosing resonance other than at an operating frequency. For example,
you can design a 40/80 trapped antenna that uses a trap resonant
around 6MHz.

Such a configuration requires a bit more complex explanation than the
trap "cutting off" part of the antenna (which it doesn't do anyway).

Owen
--

Owen Duffy wrote:
On Sun, 2 Jul 2006 18:43:35 -0000, "Dave" wrote:

you don't want to think 'loading coils' you want to think 'traps'. they are
two very different animals. loading coils are simply inductors that make
the wire look longer electrically so you don't need as much room. traps are
resonant circuits that exhibit a very large impedance at their tuned
frequency so that it cuts off part of the antenna. usually you use a trap
for multi-band dipoles, the first trap out from the center is tuned to cut
off the outside of the element at the highest frequency, and then at lower
frequencies it looks more like a small inductor so helps a bit as a loading
coil on the lower band.

Yes, but the trap (which is a parallel tuned circuit) doesn't need to
be resonant in one of the operating bands, and there are reasons for
choosing resonance other than at an operating frequency. For example,
you can design a 40/80 trapped antenna that uses a trap resonant
around 6MHz.

Such a configuration requires a bit more complex explanation than the
trap "cutting off" part of the antenna (which it doesn't do anyway).

I believe that the W3DZZ antenna uses this principle to get a reasonable
match on several bands with only one pair of "traps".

Roy Lewallen, W7EL

"Owen Duffy" wrote
Yes, but the trap (which is a parallel tuned circuit) doesn't need
to
be resonant in one of the operating bands, and there are reasons for
choosing resonance other than at an operating frequency. For
example,
you can design a 40/80 trapped antenna that uses a trap resonant
around 6MHz.

======================================

Use program NOTATRAP. The parallel tuned circuit is not a trap at
either of the operating frequencies. Its resonant frequency lies in
between the two operating frequencies. The two operating frequencies
do not necessarily have to be harmonically related.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

"Reg Edwards" wrote
Use program NOTATRAP. The parallel tuned circuit is not a trap at
either of the operating frequencies. Its resonant frequency lies in
between the two operating frequencies. The two operating frequencies
do not necessarily have to be harmonically related.
----
.................................................. .........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........

==========================================

There is a small advantage. The whole length of the antenna is used
at both frequencies. Furthermore, the antenna can be used multi-band
at frequencies higher than the lowest design frequency. Including the
frequency at which the trap is resonant if it lies in an amateur band.

But there will be some peculiar drive-point impedances presented to
the transmission line plus tuner. For multiband use one might just as
well remove the trap which is not a trap and use the antenna simply as
a random length wire.
----
Reg