Wednesday, August 22, 2007

TW2010 company response and vertical thinking

I wrote to the TW Antenna people last week looking for some basic information about the antenna. When I didn't get a response to the web form, I dropped an email message to them a few days later. The company got back to me late last night. Here is my questions followed by their response.

Very interesting. Looks like a nicer execution of the Force-12 Sigma-5 design. Does the antenna and base all fit in the travel bag? How much current does the electronics draw? (The Sigma-5 draws about 100mA to energize the relays.) If the unit is unpowered, does it "default" to the 20m band? Thank you for your time. -- Scott (NE1RD)

TW Antennas replies:

Hi Scott,

Sorry for the belated reply. We've undergone a move and a hamfest, and are just now getting caught back up to speed. It's been a mess trying to get our communications infrastructure back up since the move.

We designed the travel bag to carry everything related to the antenna (controller, cables, stand, etc).

The electronics draw about 200mA, as it has a microprocessor and LED's in addition to the relays.

Yes, the switching array will default to 20m when un-powered.
NOTE: It was suggested to us that latching relays could be used so the antenna remains on a band until it is specifically switched to another. Also, current consumption would drop on average, as the relays would not have to stay energized all the time. We're currently looking into this option as a possibility for the future.

Thanks for your interest and kind words. If you have any other questions, please do not hesitate to contact us.

TW Antennas

As Jeff (KE9V) pointed out in his post Little Store Bought Antennas this is still quite an expensive antenna. And, it is true as Jeff says that "shortened" antennas are a compromise, but perhaps not as much as one might think. Vertical dipole antennas are very effective and generally have good low radiation angle patterns which is great for DX. Switched Vertical Dipole Antennas (SVDAs) have been used on many lightweight DXpeditions with great success. They require no radial system and can be made fairly resilient to the wind and elements as evidenced by their record of service on DXpeditions like VP8GEO, VP8THU and many others.
The "shortened" part of this is accomplished by adding a capacity hat to both ends of the antenna and then applying a matching stub to null out that capacitive reactance. (The stub may actually be rolled up into a coil so it fits in the box, but you get the idea.) The book The Short Vertcal Antenna and Ground Radial (Jerry Sevick, W2FMI) discusses this idea extensively and found even very short antennas that have been properly designed can perform admirably.
My intention is to do computer modeling on all these antennas by the end of the year, but my crude back-of-the-envelope thinking goes like this: in a regular 1/2-wave center-fed most of the radiation comes from the middle 1/3 of the antenna. This is the high-current area. As we move outward towards the antenna's end current drops and voltage increases until we find very little signal being radiated at all. A Droopy dipole (a dipole hung such that some length near the ends is allowed to droop towards the ground) perform about as well as their flat-top cousins because there isn't much going on at the far end of the antenna.
The shortened vertical just rolls up that droopy part into a capacity hat. The length of antenna that is doing the vast majority of the work is still there. Further, though the radiation resistance (the part that does the good work for us) is lower in a shorter antenna, you can make up for some of this by making the radiator larger. Instead of using a wire 1/8-inch in size as you might on a dipole, the radiator for one of these verticals is one-inch in diameter. Even if the radiation resistance is only 25 ohms, a 2:1 transformer gives you a great match.
In these multiband antennas like the Force-12 Sigma-5 or new TW-2010 you still have loading coils for most bands which will reduce the efficiency. But, the other thing I'm interested in is weight, size, and versatility. Five single band antennas requires 5 runs of coax. A multiband antenna covering 5 bands requires one run of coax. I care about weight and size. It isn't just the antenna; you must also account for all the components that antenna system demands.
I erect a 40m full-sized vertical with two (or more) elevated radials as part of my antenna compliment. This antenna is also very good on 15m. If I can use a second story balcony, I am able to use a simple 20 foot fishing pole to hold this up. It is cheap, light, and very effective! I also erect a full-sized vertical for 80m suspended from a 33-foot mast. At this point in the solar cycle, antennas for 30m, 20m, and 17m give you pretty good coverage for bands that are likely to be open. Those can easily be done with fishing poles if you wish to have 1/4 verticals.
But, if you want to have vertical dipoles with an elevated feed point antenna systems like the Buddipole, Force-12 Sigma-5, and TW2010 provide a small, robust package for these antenna designs. And, when the Sun starts giving us spots again, the ability to have five bands on one piece of coax starts looking very attractive to me. $700 attractive? I'm not sure. That I need to think about. {grin}

I might disappear for a few days. I have a deadline at work and the Lowell Spinners begin a long home-stand. After that, it will be time to get serious about V4 planning!


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