Sunday, March 17, 2013

Introducing Three Short 40 Meter Verticals

My original plan was to delay installing an antenna for 40 meters until later this year. The purpose being to focus on the daylight-loving bands for the near term and then address the low bands in time for fall. Unlike the 1980s I do not spend the majority of my leisure time on amateur radio so I need to set priorities.

With this blog acting somewhat of a diary of my return to the hobby after many years I ought to do things in chronological order. Except that the time I spend designing antennas is not calendar-aligned with the time I will be building and erecting antennas. While still in the grip of winter I am focused on design. It's also a lot less effort than building antennas!

Although I have not yet discussed my plans for the high HF bands, I do have a favoured design in hand. I will come to that in the coming week or two (maybe). It is why I have been recently looking more closely at 40 meters, a favourite of mine. I thought it worth some effort to evaluate short, but not too short, antennas, comparing them to a delta loop.

There is a full-sized delta loop conveniently coiled up in my basement that needs only a mast to be hoisted into the air. I scanned my 40 meter log from 1984 to 1990 to recall how it performed. I can see that it was very competitive with 100 watts, and even better with a kilowatt. The list of rare ones from around the globe filled the pages. It first went up with a mast made of scrap aluminum I scrounged from another's toppled tower, then moved to the 20 meter tower when it went up in 1985.

I am tempted to be lazy and simply reinstall it. Then I decided it wouldn't hurt to evaluate other designs, if only to affirm that my choice was the right one. Now that I've done some modeling I am inclined to reconsider. Another thing at the back of my mind is to avoid the visual impact of a delta loop, and thus hopefully reduce the attention of neighbours.

Over the next week or so I will look at each antenna in turn. The short verticals I eventually selected for evaluation are as follows. Each requires no more than the single support plus guys that the delta loop requires.
  • Lazy-H vertical dipole
  • Linear-loaded vertical dipole
  • Linear-loaded, full-wave loop
Short antennas designs are often controversial though there is no reason for it nowadays since it is quite easy to model them and compare. We no longer need to erect them and pray, and then (of course) tell everyone how great that one antenna is while avoiding substantive comparisons which are in any case difficult to arrange and accomplish (convenient that).

There are some design considerations that make the goal of good DX performance achievable in a short low-band vertical antenna:
  • Capacitive loading (linear or hats). These are low-loss loading elements when installed with good dielectric mounts.
  • No coils! This includes traps. Coils as antenna shorteners work where the current is high, which is also where they will dissipate the most power.
  • Avoid a too-low raw (unmatched) feed point impedance, and aim to get it near to the transmission line impedance. Low antenna impedance and matching networks introduce resistance losses. It is therefore important to model the antenna with real (lossy) conductors, not ideal (loss-less) elements to measure its performance.
  • Get the points of maximum antenna current as far as possible from ground and other conductors in the vicinity. Do this to reduce the inevitable vertical-polarization ground losses in both the near and far fields, and to get the radiating parts of the antenna above local obstacles.
SWR is not on this list nor should it be. Any antenna can be matched by suitable feed system or, less desirable, with a tuner in the shack. The SWR does not indicate anything about an antenna's performance other than the risk of matching losses if you are not careful.

That is why I called this blog "Pattern and Match": first get the antenna pattern to where you want it, then, and only then, match it. A beautiful 1:1 SWR is worthless if the antenna is not launching those precious watts where you want them. As it is often said, if you want a perfect match buy a dummy load. The entire feed system from antenna feed point to the rig has only two responsibilities: to keep the transmitter happy; and to minimize losses due to feed line attenuation and radiation.

For example, if I find a tweak to my antenna that reduces environment or conductor losses by 3 db I am only too willing to trade 1 db of coax attenuation due to a high SWR. The reverse is also true: if I can get a perfect match at the cost of driving all those watts into the ground (or a neighbour's house) it is a dreadful decision. Never judge an antenna by the sole criterion of SWR.

With all of that out of the way I will as promised discuss each of the listed antennas in turn over the next few blog posts. I'll provide modelling detail and comparisons, and what I like and dislike about each. You can refer back to the criteria I set out for my antennas in this earlier post.

No comments:

Post a Comment

All comments are moderated, and should appear within one day of submission.