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An update, Projects and a Hackerspace

  Well, I sure have let this blog go quite a bit.  I have completed a few projects since I last updated this blog though none of them are the ones I said I was going to.  Instead I have built an SWR meter, a workbench, an animated edge-lit Christmas card, started experimenting with etching my own PCBs and become involved in what may be the beginnings of a new hackerspace right here in Toledo. I don't want to get even farther behind so I think I will just write about one thing tonight and work on catching up over the next few days.

  Here's how the SWR meter came about.  My mobile antenna is a Diamond HV7A which does 70cm, 2m, 6m and has a loading coil to allow it to do 10m.  I like this antenna a lot but it is rather large and catches a lot of wind on the roof of my Jeep.  I thought it might be nice to get a small dual band antenna for use in the winter when 10 and 6m are mostly quiet anyway.  I had another antenna, an Opek which was originally almost identical to my HV7A but I broke it by driving under a Taco Bell awning.  It still is intact from the base past the first coil and it still gets a decent SWR on 70cm. I haven't really tried out it's performance on 70cm much since the accident though to verify it actually works.  I just tried the SWR.  Anyway, I thought that if I added a longer whip above the coil I might get 2m to work and have my dual band antenna that way.  I tried straightening out an old coat hanger and put that in the coil to be the top whip.  Then I started checking the SWR, trimming a bit off of the top and checking it again.  Unfortunately I discovered that it does not work this way.  No matter what length I made it I still could not get a decent match.

  I also discovered that one of my SWR meters is broken.  I had two meters, one which is for HF-2m and one which was 2m-70cm.  The 2m-70cm works very intermittently.  I took it apart and found horrible solder joints.  Reheating them didn't help though.  Then I discovered a hairline crack in the body of the meter.  I think that is probably the problem.  The meter I had was pretty cheap, it is a Workman SX-144/430.  It also has pretty bad reviews for the 1.25 meter band.

  I decided I did not want to buy another Workman SWR meter but I also didn't want to spend much.  I went to a few hamfests but all the meters I found in the price range I wanted to spend were more Workman ones.  Finally I decided to look for project details online to build one.  I found this one.  It's basically a Wheatstone bridge which uses the transmitter as a power source and the antenna as the 'resistance' to be measured.  In this way it can tell you how close you are to a 50 ohm match.  The downside is that it dissipates 3/4 of your signal.  That means you can only use a couple of watts or you can burn up the resistors inside.  This really isn't all that bad because it also means you are much less likely to interfere with anyone else while tuning up.

  I built my swr meter in two parts.  I put all of the RF components in a Pamona box which I picked up at a Hamfest a couple of years ago for a dollar. I only used one meter and added a switch to select either the calibration current or the meter current.  I mounted it on a panel made from an old PC board and mounted that inside an old lunchbox. By keeping it as two pieces I can swap out the RF part for one with SO-239, N or any other connectors I need.  I may even try making a 75 ohm version.  I thought when I finally start building transmitters it might be nice to design a few for 75 ohm antennas instead of 50 because then I could use some of the old CATV cable I have laying around.  I calibrated it using a set of Precision SWR Mismatches.  I made mine with BNC connectors and used simple L-shaped pieces of brass instead of cutting out the pattern ad5x used. I left space inside the lunchbox on either side of the panel.  This way I can keep the RF box and the SWR mismatches inside.

  Because of the low power requirement I have only tested my swr meter using my HT, a VX-7R.  That means I could only test it on 6, 2, 1.25 meters and 70cm.  Using my precision mismatches there is some variation between the bands but they are actually quite close from 6m through 1.25m so I believe it is accurate across that range.  I suspect it would be accurate on any frequency below 6m as well but I haven't had the chance to try that yet. Unfortunately it is way off on 70cm.  I guess this means I still do not have a decent SWR meter for that band.  I used the same shottky diodes that Manfred Mornhinweg used in the higher frequency versions of his meter but I also used some cheap resistors which I got out of a RadioShack variety pack.  I am thinking now that they might have too much inductance.  That or I don't have enough separation between the diodes and ground.  For now I am just going to be happy with it as it is though and if I really need something for 70cm I will borrow it.

  Eventually I may build a new RF circuit for the meter using the same type of resistors that the SWR Mismatches use.  Maybe this would get me up to 70cm and beyond.  It would also up my power handling ability so that I could use the 20 and 40 meter 5 watt QRP rigs that I have with it without opening them up to lower the power.  Another idea I might try out is to replace the meter with an Arduino and an LCD.  This way I can just calculate the ratio between the two signals and display that all in one step rather than having to adjust the calibration.  I chose not to do that the first time because I liked having a passive device that didn't need it's own battery but I think that might be more convenient.


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