Moxons are also cheap, light (3 lbs on six), low profile, broad-banded and can be made multiband and multi-element if desired.
Don't forget to lay eyes on a model for that! I think some of the multi-band and extra-element Moxon designs that are available (for example, on the Moxon Antenna Project page) haven't really been well tested and modeled compared to the basic two element W4RNL-optimized version. Do the "multiband and extra element" guys show the plots?
I'm guessing that the Moxon has come into its own because the antenna really shines in computer modeling.
The close coupling afforded by the tails is not just a space saver. It's important to the good operation of the antenna. I think the "Moxon Renaissance" has a few factors, and the excellent F/R is one of them, but don't get too lured in by that either. The excellent F/R makes it seem very beam-y. And man, it saved my 6m operation a couple times in an apartment. But honestly, front-to-rear isn't even the most important thing for reception! it really depends on
how your noise is distributed.
If your noise is uniformly distributed from all azimuth and elevation angles and your antenna is highly efficient, then the best measure of your reception improvement is the antenna gain. It's not really the gain, per se, but the
reduction in signal in all directions except the one you want to listen in. If the noise is coming from "everywhere", any rejection from any direction is equally useful, whether it's reduction of signal 30 degrees off boresight or 180 degrees or 120 degrees in a rear quadrant.
For poor efficiency antennas, it's important to normalize this by the overall efficiency so you can compare. The "Receiving Directivity Factor" that you can find information about on W8JI's website takes this into account. But if you're comparing antennas that all have exactly the same efficiency, the RDF and the gain at the desired azimuth/elevation will all compare the same. And if you look at the RDF of various low band arrays, you can see that excellent front-to-rear stops mattering so much once you have, I dunno, 10 or 15dB worth of it. At that point, your reception starts to be limited NOT by stuff off the back, but by the undesired stuff off the front.
Ultimately, if your interfering signals and noise are coming from everywhere, a higher gain antenna is going to hear better
because the pattern is tighter. You don't need more gain in principle for better reception, but at VHF it's easy enough to tighten the pattern while keeping efficiency high. It's really only if you have localized noise off the back when you're pointing in a desirable direction that high F/R matters. That's probably likely enough in suburbia to make a super F/R antenna worthwhile.
But it's only in that situation, noise off the back, that they would hear better than larger yagis. When you can just barely hear that JA at 330 degrees over the neighbor's TV at 310 degrees, you'll wish you had a bigger beam :-)
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The mechanical advantages are pretty good. They're cheap and light and small turning radius... all nice.
But one of the biggest advantages, IMO, is that you can toss one together that works right the first time with whatever junk you have lying around, because the Moxgen program exists. You don't have to mess with guessing how to scale to new wire or tubing size... you can just punch the numbers in and build the thing. If you build it to within a few mm, it'll just work. Here's the measured pattern of my 15m Moxon. I built it on the shack floor, strung it up. Done:
http://n3ox.net/files/15_moxon_measurement.jpg(I'm going to post that again for ya in a different thread
)
I think the smallish turning radius, the fact that they have their own design program, plus the fact that good F/R makes people feel very successful in their beam construction is a lot of what's driving the popularity of the antennas. I have four of them :-)
But, in the end, they
are similar in a lot of ways to a two element yagi, with a broad forward beam and not a whole ton of gain. So if you can, you might want to look at a bigger antenna.
I built a Moxon-spreader-esque five element wire yagi for 6m a couple seasons ago, something like a 15 foot "boom" :
http://n3ox.net/files/sixyagi_lg.jpgIt was so Moxon-esque that now the spreaders are holding that 15m Moxon instead. The 6m yagi actually worked pretty well but due to mounting constraints, I was only getting about 4dB better at low angles (would have been 6, had they been at equal heights) and I also had to beam out over the house while not quite clearing it with the 5 element antenna... and it was too close to the siding, and all kinds of stuff. All in all I decided it wasn't enough better for my purposes to keep it instead of building a better antenna for 15.
I'll probably try the principle again sometime. I think it could make a pretty good portable collapsible big yagi if I did some tweaking. It's extremely lightweight. It's not perfect: it's really easy to break the crappie pole spreaders I used, and I need to work out the details of how the connections to the support ropes might be affecting the apparent element lengths... and I want to model the sag of the first two directors to do some sensitivity analysis.
73
Dan
