I've recently made something similar. From parts of an old KLM 4 element
20m monoband yagi with half an element missing and no need for a
monoband antenna I refurbished 3 of the very tatty elements and their
centre insulators. With the help of 4NEC2 I modelled the old 20m
reflector as a 40m dipole, extending the length by dropping about 4
metres of wire from each tip. Sort of the top half of a quad loop if you
will. I did the same for 30m using a director element and the same again
for 17m with the other director. On the 30m element I dropped about 2m
of wire from each tip and for 17m I just shortened the element to get it
resonant. The only element fed is the 40m one. 30m is spaced about 30cm
in front of the 40m element and the 17m element is spaced about 24cm
behind the 40m element. The dropped wires of the 40m and 30m elements
lower the feed-point impedance a little so it's almost a perfect 50
ohms. It's a little higher on 17m but under 1.4:1 across that band. The
wires I dropped of the 40 and 30 elements are just pvc covered 20 gauge
stranded tinned copper. I added small weights though not so much as to
add to the element droop. They move around a bit but don't affect the
swr greatly.
I was careful to try and model exactly what I would end up building and
adjusting for that accordingly. The only thing I didn't do was to
compensate for the change from aluminium to pvc covered copper on the
dropped wires. The modelling and reality converged fairly well with only
2 adjustments to the dropped wire length for each element to put it
right where I wanted it.
It is on my 80 foot tower at about 50 feet right now and I will raise it
to about 65 feet soon. The plots from my AA-230 zoom analyser don't
precisely match the modeled plots. I put that down to the local
conditions, surrounding objects etc but very very close.
I worked the KH1 expedition on 40, 30, and 17, CW SSB and FT8 where
available, I think that's the definition of a working antenna :-)
Martin, HS0ZED
On 16/07/2018 11:16, Grant Saviers wrote:
You might consider these as a two band fan dipole fed at a common
point. If the tip of the 30m was 24 to 30" from the 40m that reduces
the interaction, but modeling is still advised. Make a mast/antenna
mounting plate that has one tips up and one tips down.
See ac6la.com for advice about modeling a fan.
Grant KZ1W
On 7/15/2018 20:43 PM, john@kk9a.com wrote:
I am not familiar with MMANA but this is quite easy to model using
EZNEC.
There is no need to model your exact tubing diameters, certainly
there must
be a dipole example that you can alter. I doubt that there will be much
interaction. What else is on the mast?
John KK9A
To: towertalk reflector <towertalk@contesting.com>
Subject: [TowerTalk] 40/30 antennas on same mast
From: N1BUG <paul@n1bug.com>
Date: Sun, 15 Jul 2018 21:06:11 -0400
I have been trying to model this (using MMANA) but I have a hard
time getting my head around modeling.
I have a full size 40 meter rotatable dipole which I plan to install
at 105 feet.
I want to put 30 meters on the same mast if possible. I had
considered two options but am not sure if there will be problems
with either or both.
Option 1 would be to add a 30 meter element to the existing 40 meter
rotatable dipole, making two parallel elements with one a few inches
above or below the other, common feed. I am not certain how well
this works on bands that are not widely separated in frequency.
Option 2 would be to put a full size 30 meter rotatable dipole
higher on the mast. The maximum achievable vertical separation would
be 10 feet with 40 meters at 105 feet and 30 meters at 115 feet. I
am not convinced this won't become a two element beam pointing
straight up!
Has anyone modeled these or similar configurations?
Thanks,
Paul N1BUG
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