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Field Day 2001 Post-Mortem Part Two

Last month we talked about what went right and what went wrong. This month we'll wrap up that discussion and look ahead to possibilities for next year. Keep in mind that this is more "thinking out loud" than anything. Your input and feedback are greatly appreciated.

To quickly recap: Every year, every event -- be it Field Day or the Bike-a-Thon -- sees us working better as a group. This is a HUGE plus! As time goes on each of us better understands what we like to do and what we want to learn, and we're getting better about opening our mouths and saying it. It works best when everybody's having fun.

On the other side, we're finding out what is worth effort and what might not be. The most-mentioned example: those darn earth anchors in our locally-rocky soil. They take too long to put in and take out -- if we even can! But we want decent antennas, and they require support.

So what to do? Let's talk about types of antennas and how they relate to the Field Day situation. First, dipoles. Great antennas for radiation efficiency on transmit, not so good for directivity and noise cancellation on receive. The freespace dipole is the standard by which antenna gain is measured, and the classic "figure 8" radiation pattern isn't so small in the waist that it's useless to the sides. At the heights we're talking there are only two ends to support, with either coax or ladder line feed in the middle.

Sounds good but (seems like there's always a qualifier!): We haven't been having such great luck with dipoles except on 80 meters! This year the 40 meter dipole was a sorry performer. The coax feed checked out and electrically the antenna tested fine, but for some reason no one could hear us. They could on Doug's V-beam on 40 so we know it wasn't the receive end, it was our transmit side. 40 is THE Field Day band; not being there cost us a huge number of contacts.

One thought was the 40 dipole wasn't high enough. For an antenna to work well, you need to know how far away your target stations are and put the dipole at an appropriate height, measured in portions of wavelengths. Takeoff angle and all that. You remember on the General exam where you looked at lobe sizes at different heights above ground? The rules of thumb are: One wavelength and higher is good for DX, 3/4 wave up is good for all-round use, less than 1/2 wave up is good for "local use," within 800 miles or so. I've not found these guidelines to be totally accurate but they get the point across -- lower antennas work better closer in, higher antennas work better farther out. Some say these guidelines are true for any horizontal antenna (dipoles, loops etc) while others say the shape of the antenna helps dictate the takeoff angle.

Vertical antennas have a relatively low takeoff angle so in theory work well for DXing. This year we found they didn't work so well for Field Day as they gave very spotty coverage in nearby states! At one point we had California coming in fine but nothing between here and there, while listening on our 40 dipole with a tuner showed many stations running. Verticals are sometimes called "equally lossy in all directions" because of their radiation pattern and the tendency of the ground to soak up power. This year they worked great DX at night off car roofs in the rain but fell short working the States in the event.

Yagis are nice as long as they point at the other station! High-gain antennas are necessary on 6, 2 and 440, maybe we can put these light antennas and rotor on a pole that doesn't need earth anchors. I'm most concerned about HF. Low-gain fixed HF yagis are OK for about 90 degrees of coverage but miss a lot. They sure hear well in that 90 degree window though! Supporting HF beams requires good poles at a minimum, and high wind loads require towers.

So what to do? Jim WK2K talked about the merits of horizontal loops after Field Day. Think of a big square of wire, one wavelength long, held up off the ground at the corners. Loops tend to cancel noise (good!) and work well close-in when mounted low (again good in this application). They don't weigh much and can be held up by poles on the corners. Jim reported that 40 and 80 loops have worked out very well at previous Field Days, maybe it's time we try them again.

Problems? Well, size and interference are the only two real issues. Several local hams have vertical loops or quads with several bands making a "spider web" of an antenna. Suppose we did this with 40 and 80 loops, where the 40 loop was inside the 80 loop. Could we transmit and receive on both bands at once? Probably not -- we'd probably overload the receiver on the band that wasn't transmitting because of proximity to the transmitting antenna. Size-wise they take up some space on 40 and especially 80m. We'd need more coax to put up separate loops on each band as they take up room.

What to do? Well, it looks like a mix of antennas is in order. Maybe next year instead of a V-beam we'll put up a 40m loop, and feed the 80m dipole with just ladder line instead of a hodge-podge of coax and ladder line. Maybe we'll put up vertical loops for 10m, 15m and 20m and a horizontal loop for 40m. I really don't know, I haven't worked out a combination I really like yet. I keep wanting to put up a beam for one band to simplify interference issues. I do know that driving stakes for pole guy ropes is much faster and easier than drilling earth anchors.

What do you think? Let me know and let's use the winter to build and test some antennas!

73, N2VR

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