[NLRS] 6m rover antennas

[Jon Platt]

Hi John, sound like you are having some fun plotting & planning your rover
set-up.   Here are some random thoughts from the cheap seats:

1.   I really haven't used EZNEC to optimize yagi design too much .... as
you mention its a bit tedious.  Also most of the cut & paste formulas out of
the more modern handbooks, etc., are pretty darn good .... they have already
been optimized.    Regarding using EZNEC in this capacity, here are some
things to remember.
A.  When comparing one design to another, make sure that you are always
using the same ground model.   For my email note a while back on the 3
element NBS at 10', that was modeled in real ground mode, High Accuracy
model, with a ground description of "good".   The NBS model that I was using
was the one that is in the EZNEC library called NBSYAGI.
B.  Be away of element taper issues.   If your design is going to use
different diameter tubing to taper, then you need to be aware of this in the
EZNEC emulation.   There is a discussion on tapering in the manual.

2.  Regarding optimization, its my understanding that all yagi designs are a
compromise between maximum forward gain, maximum front-to-back ratio, and
bandwidth, so when you design a yagi, you need to determine what your
optimization priority is.   Typically, for a given boom length, as gain goes
up, the bandwidth tends to decrease and the feedpoint impedance drops.

3.  The feed method also has something to do with bandwidth.   If you are
using a 50 ohm direct feed you will find that as you maximize forward gain
that the feedpoint impedance will drop to around 12 to 22 ohms or so
resulting in a poor direct match.   You can play around with the element
lengths, especially with a "long boom" design (like 3 elements on a 12'
boom) and get a pretty good match for a direct 50 ohm feed, but you will
find that the forward gain is compromised some .... I suspect that is what
you've found in your design as the SWR bandwidth is good, but forward gain
is on par with an optimized 3 element.   For simplicity and performance, its
hard to beat a gamma match ..... if you are looking for a good SWR from
50.100 to 5.300, that should be very attainable using a gamma match even on
an gain optimized 50 MHz yagi.

4.  The performance difference between a 4 ele yagi on say a12' boom and a 3
ele yagi on say an 8' boom is more than likely not observable 98% of the
time by the station on the other end.  On the other hand, when you are in
EN24 and the wind is blowing at 25 mph (on a calm day), the mechanical
issues associated with a 4 ele on a 12' boom vs a 3 ele on a 8' boom will be
very very apparent.   My suggestion is to go with a light weight 3 element
design to optimize (ie reduce) mechanical issues (ie, weight & windloading).
In other words, my suggestion is to optimize weight & windload rather than
optimize forward gain and bandwidth.

So in summary, my suggestion is that you look at designing a super
lightweight 3 ele design (in order to reduce weight & windloading), use a
handbook based design, and incorporate a gamma match.   Easy to put up, good
performance at 20', and a 1:1 match at 50.125 with good bandwidth to cover
the rest of the band.   Let us know on what you decide to do .... hey, how
about a presentation at Au'03 on designing high performance lightweight
yagis for roving ??   I think a lot of people would be interested in this
subject for both roving and portable work .... I know I would.

73, Jon
W0ZQ