I am not familiar with all the limitations of MMANA, since I mainly use
EZNEC. However, the procedure would be to include the towers into the
model. Use segments that approximate the diameter of the towers. Also
include models of all the antennas on the towers. For this purpose, you
don't have to do accurate models of the antennas, just do enough to
approximate their top loading affect. You may run into problems where
large diameter segments intersect small diameter segments, and have to
do some tapering.
The tricky part is modeling the wires that are exiting the tower. If
you do not have a good radial system attached to the bottom of the
tower, the wires that exit at the bottom of the tower will heavily
influence tower resonance. Modeling this is almost impossible.
If the tower has a good radial system, you can neglect the effect of the
wires exiting the tower, assuming the wires are connected to the base of
the tower as they should be. This can be modeled using a mininec ground
and adding a series resistor at the base of the tower to simulate ground
losses. You can look up the value of this resistor from charts that
have been published for ground losses versus number of radials. Try the
ARRL Antenna Book. I don't think that MMANA has a mininec ground option,
so you will have to model the radials. Also I'm sure MMANA will be
running NEC2, so it will not properly calculate near field ground
losses. It calculates something for near field losses, but it is far too
optimistic. So you will still have to manually add some resistive
losses to correct for this inaccuracy. It calculates far field losses
OK. The same situation exists for your 4 square elements, and the 160
You have a sufficiently complex system, that everything should be
included into the model so you can look for interactions. Be sure to
include any conductive guy wires, or any vertical feedlines that are not
taped to the tower.
A side note: When I have modeled systems such as this, in terms of the
affects on four squares or any vertical directive arrays, I have found
that there is always a significant interaction, even when the towers are
not resonant near the frequency of interest. The effect of that much
metal in close proximity is difficult to eliminate to an extent that
does not significantly effect the F/B ratio.
Adrian Rees (MW1LCR) wrote:
>First of all, many thanks for the extremely helpful advice you have all
>offered, both on and off list.
>After giving this some thought, I am going down the route of de-tuning the
>towers, and modelling the potential solution. At this point, the actual
>towers havn't been installed, although I have permission to install them.
>They MUST be in a square formation, but can be between 20 and 50 m apart.
>Each tower will support various Yagi's....So.....
>I am using MMANA to model a 4 Square on Topband, and need some help.
>Although I have a working 4 Square file that MMANA can use, I am puzzled how
>to model the effects of the metal supporting towers in MMANA.
>Now then, I plan on running a catenary wire (well rope) from the top of each
>tower to the next one. This will support a T aerial, fed against earth (or
>elevated radials). The vertical part of the T will be 80 feet, the remaining
>top part of the T will be about 40 Feet a side. This seems to work FB in
>Freespace in MMANA.
>What I need to do is to model the effects of the supporting towers. I am
>particularly interested in the effects on the radiation pattern / angle.
>The plan is to de-tune the towers with Gamma Rods with either Capacity or
>Question is, how far do I de-tune them, given that an 80 M 4 Square is
>planned as well.
>Any help ?
>Adrian Rees MW1LCR
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