Thursday, March 15, 2007

Featherweight tribander (continued)

I've gotten some very interesting feedback from my post the other day about my search for a featherweight tribander. Dave Murphy (KB1LKE) wrote to me not long after the post to challenge some of my assumptions and make some alternative suggestions.
Even before I get into this, let me say that this is good stuff. Challenge assumptions. Reconsider alternatives. Think hard about your choices and understand why you're making them, what you're getting, and what you're giving up. Dave's message had a healthy dose of all that.
I had said I was looking for a featherweight tribander and listed some reasons why I thought one might be desirable. Dave restated some of my thinking simply: "It is apparent that you want to output a little more power and have some directivity to minimize interference from certain directions. The only questions are how much power and directivity and how much are you willing to sacrifice for them." Exactly. I had also quietly buried an assumption, though, that he picked up on immediately. Do I need 3 elements on this thing? A two element yagi is also a very effective antenna which provides a good front-to-back ratio and some gain, maybe 5 dB, over a dipole. Adding that third element only gives you a small bump from there. Consider a couple of Mosley "mini-beams" suggested by Aston Lee in the comments section of this post. The data below was gleaned from the Mosley web site.
Mini-33-AMini-32-A
3 Element 2 Element
Gain dBd F/B Gain dBd F/B
10m 6.1 16 5.1 ~17
15m 4.2 13 4.5 ~17
20m 3.5 12 3.3 ~17
Weight 10 pounds 8 pounds
MSRP $468.95 $347.95

Indeed, as per the table above, that extra element does provide a little more gain on 10m and 20m, but look at 15m. The extra spacing between the elements of the 2-element antenna actually helped on 15m gain!
Why a tribander? Could a reconfigurable antenna that provided different bands be as good? Aston Lee also suggested looking at
Super Antennas (W6MMA) Portable Yagi System. This is a two element antenna that can be configured for any single band 20m-6m. It weighs only 7 pounds and comes in a nice carrying case. I remember seeing this at Dayton last year and being intrigued for a time, but in the end losing interest. The main reason for this is another unstated requirement that should have made the previous list.
When you consider the weight of an antenna system for a 100 Pound DXpedition you also need to consider the weight and bulk of its feed line. If you need 100 feet of RG8X to feed a given antenna, that coax run will add 4 pounds to your pack. If you have three mono band antennas that need such a run, that would add 12 pounds. One of the appeals of a triband antenna is that I get to feed that single antenna with a single piece of coax, theoretically saving you weight.
The other reason why the collection of configurable mono-banders did not seem like a great deal to me has to do with what I already have in my antenna collection. I have already purchased or designed mono-band antennas, collapsable masts and fishing poles hanging wires for the low bands, Buddipole, Buddistick, or MP-1 for the higher bands, and some other weird things I've picked up along the way just to add to the variety. I don't need any more single-band antennas. Further, if I'm going to add complexity of assembly in the field to the equation, I want to have a significant payback for that. Gain, a nice F/B ratio, and three bands from one wire sounds like a nice trade-off. Anything less does not.
Build or buy? Returning to Dave's message for a moment, he suggested that I look into some articles Cebik has on his web site. (This is always recommended. If you love antennas and haven't been to W4NRL's site, you're missing a lot!) I am a "tinkerer" by nature and can be easily enticed into wandering off into a very interesting distraction. I must not allow that to happen this time. My goal is to have whatever system chosen to be in place this Spring. I would like to have ample experience with it before it makes a trip to St. Kitts.
Will it fit?! I listed "breaks down small" in my criteria in my original post. I could, I guess, find some hard-sided ski bag to haul around 6 foot pieces, but I would rather not. I have a very nice hard sized golf bag that was served me well and would very much like to continue to use that for carrying my antenna parts. Further, even if I get a tribander, I will still need to carry other long items such as the DK9SQ mast, fishing poles, and my trusty big Buddipole system. This, too, becomes a "build vs. buy" alternative. If I buy an antenna, can I get the manufacturer (or somebody else with the correct tools) to chop it up so it would fit in my bag?
As mentioned before, I was considering the Hy-Gain TH-3JRS. At 21 pounds, it was on the heavy-end of my criteria, but it looked to be a good performer. To be honest, I didn't think anybody made what I was looking for: small, light, and low-power rated (to enable these savings). I was wrong. Look again at the Mosley specifications in the table above. These antennas are 8 and 10 pounds respectively for the 2- and 3-element models!
Intrigued, I called Mosley this afternoon and spoke with Gary. There are stories on the Mosley news page about DXpeditioners ordering antennas cut to size. Could he do that for me? Yes! There would be some additional charges (which we discussed and sound reasonable to me), and assembly time would probably be a little longer than the "stock unit", but I could order the antenna the way I wanted it (short pieces, please) and he'd do everything within his power to accommodate me. That's service!
I now need to decide if I'm really ready to drop another big bucket of money on an antenna. Also, do I get the 2-element or 3-element model? In any case, I believe I'm a few steps closer to my featherweight tribander.

2 Comments:

Blogger Steve Weinert said...

One wonders if you could phase three or four Buddipoles in an array.

This would standardize the gear used, reduce the issues of getting a Yagi type antenna up & rotating it.

Yet it would give you the selectivity, gain, compactness and utility you are looking for.

The only real hassle would be either working out a compromise in spacing that works reasonably well for several bands, or making the spacing changes & feed-line adjustments to reconfigure when significantly changing bands.

One wishes the antennas could be linked wirelessly and the phasing/delays all done in the control box to simplify reconfigurations for various bands (hint hint).

Glad you are mended well enough to be back writing!

73

Steve
K9ZW

March 15, 2007 9:41 PM  
Blogger Ashton Lee said...

Scott

I've also built one of these... http://www.moxonantennaproject.com/m3kxz/M3KXZ_VERTICAL.htm ... hasn't been warm enough around here to try it out yet. But you get ultra light weight, directionality, gain, but not multi-band performance. I built 3 since they are cheap and easy to hang-- one for 20, one for 17 and one for 15 meters.

Moxons are also great when used as horizontal antennas... but I think the idea of using them as verticals is brilliant.

The 33" fishing poles are one way to hang these as verticals. The other way is to use light PVC pipes to hold the shorter horizontal sides One on top and one on the bottom) and hang the whole thing from a tree, under a bit of tension to keep it all lined up.

W9ASH
Ash

March 16, 2007 6:12 PM  

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