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Author Topic: 80/40 loaded/trapped dipole  (Read 12328 times)

Posts: 75

« on: May 06, 2013, 07:46:31 AM »

I'd like to make a short dipole for 80 and 40 like the MFJ 17758.  It has coils that (purportedly) act as traps for 40 and loading coils for 80.  I could just buy the antenna from MFJ but I'd rather build it and understand how it works.  MFJ says the antenna is 85 feet long in total. 

I have a decent antenna analyzer (RigExpert AA-54) if that would help. 


Posts: 3270

« Reply #1 on: May 06, 2013, 08:26:37 AM »

August 2012 Q&A on this related antenna subject

The 40/80-meter dipole (trapped, loading coils, fan) was very popular from 1950 to 1980 with Novice operators.  
The antenna would often function well on 15 meters, using the 40-meter section as a 3/2-wavelength dipole.

Designing a Shortened Antenna
QST Magazine, October 2003, Pages 28-32
by Luiz Duarte Lopes, CT1EOJ

William J. Lattin, W4JRW
Multiband antenna - December 1960 and April 1961 QST magazine

Bill Fanckboner, W9INN (sk) designed, built, and sold a number of shorted wire antennas
until 2004, when he passed and his family shuttered the business.

All of Bill's antenna designs used only 2 dipoles/elements (fan dipole).
Bill's inline coils (Resonators) are 2" OD forms (dark gray) and coil length 2-1/2"
with the plastic form longer for wire attachment (5 to 12 inches common)
One foot wooden dowels are used to separate the two dipoles.

W9INN Sloper - Instructions

W9INN - MPD-5C Max

IF you desire DIY, start your design with the K7MEM Short Dipole Calculator.

Coil Inductor Calculator

Follow diagrams for Building the Resonators, per Bill's diagrams.

Homebrew Your Own Inductors
by Robert H. Johns, W3JIP,
QST, Aug 1997, p 35

1-1/2 inch Schedule 40 PVC (gray) Electrical conduit has a 1.9 inch O.D.
Very good form for winding the 2 inch loading coils (this is what most builders use).

Alpha-Delta and now MFJ later commercial antenna designs.

Hy-Power Antenna Company in Bethlehem, PA offers the loading coil and parts.

Hy Power Antenna Company
2028 Riverside Drive
Bethlehem, PA   18015
Phone: 610-317-9779
« Last Edit: May 06, 2013, 09:04:55 AM by W9GB » Logged

Posts: 75

« Reply #2 on: May 06, 2013, 08:41:49 AM »

Thanks for the info.  Your W9INN sloper and WPD-5C-78 links are dead. 

Posts: 3270

« Reply #3 on: May 06, 2013, 08:49:39 AM »

I was repairing those links, when you posted.
All links work, and I added additional DIY Builder resources / tools !


Posts: 17791

« Reply #4 on: May 06, 2013, 12:23:39 PM »

There are at least two different methods of achieving 40/80m operation by just adding
inductors to an antenna:  while they look similar they are based on different behavior.

It would seem that the 80m loading coils might have enough inductance to look like a
choke on 40m, but that rarely is the case in practice.  (It does work for combinations
with a wider split such as 20m / 80m.)  So to make the coil act like a trap you have
to design it to be parallel resonant due to the stray capacitance.  That means that the
turns spacing, wire size, and former material are all critical to proper operation.  You
can use this inductance calculator to help you get close, because it reports the
self-resonant frequency:

In that case you choose a coil design that is self-resonant on 40m to act like a trap,
then add however much wire you need to the end to get the 80m resonance where you
want it.

Note that while such antennas are advertised as "no lossy traps", because of the self-
resonance of the coils they can actually have more loss than a well-designed trap.
In my experience, using an explicit trap is usually easier than trying to get the right
self-resonant property of the coil.

The other approach doesn't rely on the self-resonance of the coil, but rather is carefully
placed and adjusted so that the wire is at 3/4 wave resonance on 40m, while simultaneously
providing an 80m resonance.  This approach has a lot more interaction between the resonant
frequencies in the two bands, and tends to have a narrower operating bandwidth on 40m.

The free Demo version of EZNEC should be sufficient to model these, at least as half the
antenna fed against ground, and that may be helpful to understand the differences.

In practice, however, I have found design, construction and adjustment of standard trap
antennas to require less work than either of these two methods.  Something like this:

With real coils, neither of these two methods is likely to work any better than the trap
version, and by adding an explicit capacitor across the coil rather than counting on the
value of stray capacitance to form the trap it makes a more reproducible design.

Posts: 75

« Reply #5 on: May 06, 2013, 02:08:27 PM »

WB6BYU, if I just use simple traps then it will still be full sized for 80 meters then, right?

Posts: 17791

« Reply #6 on: May 06, 2013, 03:02:12 PM »

No, the traps have inductance on 80m, and will shorten the antenna on that band
as well.  The amount of shortening depends on the L/C ratio of the traps, so high L
and low C are often used to decrease the length on 80m.  There is a version of the
coaxial traps that appeared in QST (I'll have to look up the issue) where they used
a double winding of just the coax center conductor:  that doubled the L and cut the
capacitance in half, and they managed to fit a design for 40 / 80 / 160m in the space
of a normal 80m dipole.  (My approach to the same end is to put an 80m dipole in
parallel with a 40 / 160m element.)

But there is nothing that says you have to achieve all the 80m shortening only from
the 40m trap:  you can add a separate 80m loading coil outboard of the trap and
make the overall length shorter, and/or you can add a coil inside the 40m traps where
it shortens the length on both bands.  There is an infinite range of designs, adjusting
the positions and values of the loading coils and traps along with the wire lengths to
get an antenna that fits your needs.

Posts: 801

« Reply #7 on: May 09, 2013, 11:26:07 AM »

I have a 160/80/40 W9INN dipole at about 50 ft. Excellent SWR at centers of resonance. However, the bandwidths on 80m and 160m are very narrow.  Also, this dipole is some 5-10db down on 160/80 than an 80m dipole fed by ladderline at 60ft. 

I feel that the efforts to tune a multiband dipole with coils is not worth it. Especially when one has access to automatic antenna tuners. 

Ignacy, NO9E
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