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Cheap antenna tuning notes

2011/04/22 1 comment

After hearing of the problems from the McARCS antenna building project with the WA5VJB Cheap Antennas, I decided to build a few more myself.   I have built a number of these antennas for bands from two meters to 1296.  I have not had any problem with any of them.  I also made  a get together day plan with Carl (KB6ZST).. 

It seems as tho the major problems were already identified, namely too small of wire, and the insulation left on the wire.  While the antenna in this mode was just below the two meter band, it would not tune up.

Carl borrowed Steve’s (KJ6EIF) antenna analyzer and I brought mine also, along with two of the antennas I made.  Mine were with one eighth inch bronze rod and were pretty close.  Carl found with the analyzer he could play and prune the antenna into the proper range ((see his Nibbling notes on this blog)).

This last week I played with mine at home.  One antenna tuned just right for where I wanted it–144.200–and the other was high by about 550 khz.  I tried to lengthen the driven element single rod piece with a short section of tubing, but it did not move the resonant frequency swr much.  I moved the tubing to one end of the first director and the slightly long now element made the antenna come alive.  Easily went to one to one swr at my required frequency.  That first director has a REAL BIG effect on the impedance and matching of the antenna.  I have read about this before but never really noticed the amount of the effect.  Guess I have been very lucky in my past antennas!!

BTW, my antenna showed very close to a one to one at 144.200 ((my design frequency)) and a two to one swr band width of plus/minus one and a half mhz.

Anyway, if you build one of these antennas with the WA5VJB specifications, it should come close.  To first check the swr, move the coax connection location slightly.  Then maybe try a slight length adjustment of the driven element or the opening/closing of the driven part of the loop.  Then you could try a shortening or lengthing of the first director.

Please note that there are differences of length for different sizes of rod!!!  This can affect the frequency of operation.  Even on two meters there is an adjustment to be made between the ssb/cw and the fm portions of the bands.

Good luck and happy antenna work.

A 2 Meter J-Pole Jig

Last week’s McARCs meeting/workshop in Mendocino was a lot of fun. The turn-out was good too — better than anticipated. I’d like to thank all of those who made this meeting possible and who helped out with the antenna workshop.

One of the interesting demonstrations was Alan’s (WA6JBK) j-pole jig which he uses to rapidly construct 2 meter J-poles from ordinary copper pipe (same as used for home plumbing).  Notice how Alan decouples the coax by passing it within the lower segment of the antenna pipe.

2M J-pole jig

WA6JBK's J-pole jig for building 2M antennas

I use a similar J-pole made by Len, WA6KLK, and they’re strong, easy to setup antennas that work quite well. Here’s how to make a J-pole — but note the difference in how the coax is attached. (It would be fun to electrically compare the difference in the two decoupling methods: wire loop vs. Alan’s copper sleeve.)

Another antenna construction project presented by Steve, KJ6EIF was a hands-on build your own super cheap 2M Yagi-Uda antenna (for only $5). The design is based on WA5VJB’s cheap yagi. Steve made one about a year ago and it tested out pretty well.

We made the Yagi antennas from lengths of ordinary #12 house wiring attached to wooden garden stakes. The feed method was a simple half-folded dipole (no gamma match or balun needed.) All-in-all it took about an hour to build each antenna.

Unfortunately… after construction, we discovered that our antenna resonance rang up around 143-144 MHz, quite a lot lower than we had intended. We experimented around a bit with shortening the driven element and shunting its folded-side, but neither changed the frequency by much (although the shunt did give better control over SWR.) We suspect the culprit to be the insulation we left on the directors. We kept it on to make the elements stronger in the wind, but insulation will affect the velocity factor, making those elements electrically longer than they should be. On a Yagi-Uda, you can get by with directors that are a bit too short, but if you make them too long, the phasing of the array is quickly spoiled.  Well, not a problem… we will try snipping down the directors to see if we can get the Yagi where we want it. (We’ll let you know how this works out.)

Thanks again to all of those who made this workshop a fun and educational McARCs meeting.

Tale of the Rechargeable Battery

There are strange things done in the midnight sun
By the men who moil for gold;
The Arctic trails have their secret tales
That would make your blood run cold;
The Northern Lights have seen queer sights,
But the queerest that’s ever been said,
Was that night on the marge of Lake Lebarge
My HT battery went dead.

(My apologies to Robert W. Service and his Cremation of Sam McGee; a favorite of mine, and somehow fitting here.)

I think we all have rechargeable battery tales, don’t we? For nearly four decades now, ever since my father bought a military surplus ni-cad battery array, I’ve been in a constant battle with batteries. There is perhaps some irony in the root of those two words, battle and battery, which are French in origin.

There are many myths, not-so-sound advisories, and nutty ideas about rechargeables. These days, I ignore them all and go with what works well for me.

Here are a few of my conclusions:

  • Keep your batteries “topped off”. In other words, recharge often and recharge fully. How often?  Every day, if possible, but for sure never go longer than a month. Have you noticed that those solar lights in your yard will run for two or more years, often 700-1000 cycles, because they diligently follow this rule (of course, they have no choice.)
  • Use a good quality charger. If it’s not providing a complete charge, or if it charges with too much current (battery gets really hot, causing internal evaporation), you need to find a better charger.
  • Avoid complete discharge, if possible. Sure, you will get all kinds of advice on this point. Just keep in mind that you’re using a reversible chemical reaction that emits heat and deteriorates “at the edges” of the curve: full discharge and over-charge.
  • Use a good battery tester to confirm that your battery has reached the end of its life. I don’t know of any good battery testers, so I made my own. It’s a shunted milliamp-meter that pulls nearly full current for a very brief period. This technique makes the condition of the internal resistance quite obvious.

The second point is worth expanding on. Some battery chargers are devious. The worst ones, like on my cheap Ryobi cordless drill, never stop charging, eventually drying out the electrolyte. I lost three drill batteries until I noticed their sly trick (selling more batteries.)

Most average battery chargers will allow a battery to go dead even while it’s in the charger. They may do a good job charging, but once done, the battery will begin discharging on its own. It’s aggravating to take a battery out of the charger, only to find it’s dead. (Note this will happen on many laptop computers too.)

A good charger will top off the battery with regularly timed cycles, or provide a trickle charge. However, obtaining the correct trickle charge is non-trivial, and most chargers won’t do it properly.  Actually, I don’t know of any, so if you own one (or built one) let me know.

Also, keep in mind that a dead battery pack does not mean all of its cells have gone bad. I often break open dead battery packs and test the cells separately. It’s usually just a single cell that died (1 in 6 or more) but most people replace the entire pack, costing $60-90. Another sly trick?

Anyway, I’ve written about this topic a few times in the past, and I could go on forever.  You probably have your own special tales to tell. I invite you to post them in the comment section.

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