[NLRS] 50Mhz antenna modeling question

[w0zq at aol.com]

Hi John. Roving and antennas is always a good conversation ! I think 
part of the discussion is if you are planning on stopping and doing 
some set-up, or if you are only going to rove with antennas mounted to 
the car. If you set up, you can do some more things regarding bigger 
antennas mounted higher.

6m is one of the toughest bands for rovers because it almost HF, and 
ground effects are a problem. Yes, a big yagi up high is the best, but 
what about when you only have 20 or less feet ? And what do you do when 
your a rover in corn-land and the wind is out of the south at 25 mph 
with gust to 35 mph ?

For 6m, if you look at gain at take off angles of say 5 degrees or 
less, a dipole mounted at 20' is only 1.7 dB dB down from a 3 element 
yagi mounted at 10'. It may be easier (and more reliable) to get a 6m 
dipole to 20' than a 3 element yagi to 10'. For 6m, for a rover, 
trading off windloading and weight in favor of height may be a good 
compromise. Of course, the 3 element yagi at 20' is even better !

For the higher bands, yes, long boom yagis are good for gain, but once 
again you are looking at a lot of windloading and weight, and tighter 
aiming requirements. If you are setting up and perhaps have a rotator, 
this may be OK. If you have the yagis bolted to the car and depend on 
rotating the car, this may not be so good. My experience is that medium 
boom lengths, 4 to 8', are a good compromise. YMMV.

For 6m roving, the Par Moxon's have been very popular as they do 
provide gain in a lightweight mechanical design. The key for getting a 
good signal on 6m as a rover is height, not necessarily boom length.

BTW, I think its Directive Systems that has a "rover" design for low 
band yagis with booms all under 10'.

73, Jon
W0ZQ



Matt:

Antenna modeling is one of those things I have always wanted to get 
involved with, since I am a computer geek and software developer, but 
it's been several years since I could find any time to do any 
programming for fun, and so it's been on the back burner. Which is a 
long way of saying I can't help with your actual question. So why am I 
writing at all?

I feel the need to ask a question that may impact on your whole 
process. Although I am familiar with the KISS principle (Keep It 
Simple, Stupid!), and sometimes even adhere to it, I don't see the 
reason for trying to make a two-element yagi for 6 Meter roving. Why 
not at least 3 elements? That should still allow for a very manageable 
boom length suitable for roving, and provide better gain at the same 
time. You even said you have had good luck in the past with 3 and 4 
element yagis, so why consider dropping all the way down to 2 elements?

Since I am so frustrated by trying to work rovers that I can't hear and 
that can't hear me, I advocate "lots of antenna" when roving, rather 
than "the least antenna feasible". My "perfect" rover antennas would 
all be 10 feet long so they were street legal when pointed in any 
direction, but since I haven't found a set of 10-foot antennas, and 
don't have the time now to delve into antenna modeling the way I hope 
to eventually, so can't design and build my own, I use what I can that 
comes close to 10 feet. My loopers for 902 and up, for example, are all 
the 12 foot models, rather than going down to 8 or 6 foot booms. OTOH, 
due to lack of time to fiddle with antennas, I also tend to go out with 
a full 6M5X antenna on 6M since that's the only 6M antenna I own that 
is not attached to my tower, and that's a BIG antenna by rover 
standards! And I use the rear portions of M squared antennas for the 
next 3 bands up so that they are cut off at something close to 10-12 
feet in length. I'd really rather have a set of "optimal" 10 or 12 foot 
long antennas on the 4 lower VHF/UHF bands, but I'd also rather 
occasionally go out than only dream of what could be.

Anyway, I'd consider evaluating a 3 or 4 element design before spending 
a lot of effort on the 2 element, unless I'm missing something 
fundamental here.

73 de WØJT

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