Friday, July 07, 2006

Antennas for travel: Force-12 Sigma-5

Today I'd like to just mention a few things about the Force-12 Sigma-5 antenna. I ordered this antenna about a week ago and am now awaiting its arrival. Of course I'll revisit the discussion after I've used the antenna in the field!
I thought some of you might be interested in my motivation for ordering the antenna in the first place. After all, I've just finished several lengthy posts describing the MP-1, Buddipole, and Buddistick antennas. Exactly how does this new antenna fit into the mix? So, before I tell you why I ordered it, let me tell you a little about the antenna.
The Sigma-5 antenna is a 5 band vertical antenna covering 10, 12, 15, 17, and 20m. It is rated at 1200 watts SSB (700w CW) and its makers claim it is greater than 90% efficient. That's nice, but how about size and weight (two things we care about deeply)? The antenna breaks down into 2 foot sections and weighs only 7 pounds. At this point they had my attention!
The antenna is basically a vertical dipole with capacitance hats. These capacity hats reduce the height of the unit while keeping efficiency high. The trick that the antenna uses so it can operate on multiple bands is found in the center housing where a set of relays can be energized from the wired remote switch to select between the various bands. If no relays are energized, the antenna is left in its default state as a 20m antenna.
At this point we can begin making some comparisons between the Sigma-5 and some of the other antennas I've already discussed. Here are some things the antenna has going for it:

  • Weight and size -- My Big Buddipole system is 12 pounds; my Small Buddipole system is 8 pounds. The Sigma-5 comes in a pound lighter than even the smaller Buddipole. That's nice.

  • Remote band switching -- This is an important advantage. If you are working on 20m and want to just sneak up to 17m or 15m meters to see if there is an opening, the other single-band antenna solutions would require that you reconfigure the antenna for the new band, a very time consuming process. The Sigma-5 can change bands instantly with the twist of the band select switch run from the antenna to your operating position by its 50 foot cable (which you can extend if you like).

  • Higher power handling -- I don't own an amplifier (yet) and, in fact, do much of my work QRP. But, that said, I could see a day where I'll be on an island (maybe on Cay Sal Bank) with a small 500w solid state linear for, say, 20m just to ensure we get heard on at least one band. This antenna could handle that power. The other antennas have a 150 watt or so power limit.

  • Quick assembly time -- As good as I am with the Buddipole I believe the no tools, no tuning, 5 minute assembly time for the Force-12 will beat even my best time for the Buddipole or any of those other antennas mentioned. When you are operating far from home you want to reduce any complication you can. The quick assembly time promised here is a serious advantage.

  • Built to be a vertical dipole -- I like to operate my Buddipole as a vertical dipole when near the salt water but I'm always concerned about the interaction with the aluminum mast. The Sigma-5 has no such complication since the entire thing is the radiator. That is an interesting point, in fact: about half the weight of the Buddipole package is dead weight in the form of tripod and mast. A seven pounds (less control cable and switch) of the Sigma-5 is radiator. I like that idea.

With all these kudos, you might think I'm ready to abandon the other antennas. Not so. Here is the other set of arguments against the Sigma-5:

  • Only covers 10-20m -- As we approach the bottom of the solar cycle, 10m and 12m are only a memory. Even 15m (which I worked a bunch on Field Day) is spotty at best. That means this antenna only covers two bands when the Sun is sleepy. My Buddipole, Buddistick, and MP-1 all cover down to 40m and perhaps lower with some clever hacking. We can argue efficiency all day but if an antenna can't be used on a band, it is zero percent efficient and any antenna that radiates beats it.

  • The antenna draws current -- As I said, I operate QRP in the field much of the time. I'm proud to say the radios I've built from Elecraft draw very little on receive, usually on the order of 20-40ma. The review of the antenna claims it draws only 95ma! Goodness! That's three or four times the current draw of my radio! (I plan on doing some measurements of this when I get my unit. I can't help but think latching relays would have been a better option here...)

  • Price -- At something approaching $400 delivered (I'll know when they ship it), this is no cheap antenna. I can't help but wonder if I should have gone with a small handful of monoband vertical dipoles. I'm not sure I could have made the 7 pound weight limit, but I'm sure I could have beat the price! [grin]

I could go on but you get the idea. It may not be an obvious that blowing a wad of cash on this is a good idea. I don't know myself. Still, while sitting on the water's edge while operating from Georges Island in Boston Harbor, I couldn't help but think, "this is a perfect place for a vertical like the Sigma-5." I'll let you know.
In the mean time, you can see the review from the ARRL's October 2002 QST here. The glowing review was part of the impetus to try this antenna. See if it doesn't sell you on it, too.
Tomorrow I'll discuss why a man that never fishes needs so many fishing poles.


Post a Comment

<< Home