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I put up a doublet antenna today.

Captain Kilowatt

Professional Amateur
Staff member
Apr 6, 2005
17,260
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Nova Scotia,Canada
Since the bands are starting to show some life and things have started to wind down for the winter I decided it was time to get back on the air. The only HF antenna I had up before today was an inverted L that is 30 feet high and 75 feet long and worked half decent despite having no radials. Since the WX today was fairly cold,about 10 degrees F, and I had to traipse around in a fresh layer of snow I knew the antenna would work good. Antennas put up in the winter or any adverse conditions always do. Perhaps we just think that because we appreciate when the job is finished more. :D Anyway, I constructed the basic doublet cut for a half wavelength on 80m from a roll of #12 THNN wire, fed it with 450 ohm ladder line and installed it on the tower in a basic inverted VEE fashion with the apex at about 38 feet,not great but it's all I have right now. I then wound a 4:1 current balun on an air core form from some more #12 THNN wire and connected it between the balanced feeder and about 10 feet of coax cable that goes from the shack down to the basement. I could get the balanced feeders into the basement with ease but it is a bear trying to get it up inside the outside wall so that part can wait until spring.I fired the rig up and scanned through 80m and one of the first signals I heard was 7J4AAJ. He was about 5 and 7 on the TS-820S. I was impressed and then I thought "Hmmmmm......7J4AAL? That call is familiar." A quick lookup of his callsign revealed why he was the only Asian station I was hearing on 80m at the time. Apparently although he lost what was the world's largest 80 amateur antenna in the world back in October,he still has a potent signal on the band. I would have LOVED to work him but the QRM from the west coast was too much for me to punch through.I plan to be on late tonight around the DX window 3780-3800 and see what this thing will do.

Happy New Year's one and all.


7J4AAL's Full-Size, 5-Element, 80-Meter, Rotary Beam Antenna
 

Well the antenna seems to work half decent on 80m.I'll try it on the higher bands today when they open however I do seem to have a problem on 20m with RF in the shack.It figures since 20m is my favourite band. The rig does not like it when the tuner is tuned to a perfect SWR.The plate current drops and the power output drops to about 50 watts. The CW sidetone is raspy and the voice is distorted when heard through the monitor. Time to break out the braided strap and start bonding everything to ground. I should have done that in the first place but after rebuilding the shack I forgot all about a few "minor" details. :oops:
 
The symptoms might be there if the combination of feedline and doublet puts the load at the tuner at a very high impedance.

Try adding or subtracting feedline OR doublet length. Add 8 feet to the feedline OR 16 feet to the doublet as a starting point.

The extra feedline can be managed by trying some string midway down the span and tensioning it to a tree.

I've seen people who don't understand this concept shorten the feedline and lengthen the doublet at the same time hoping for a change:blink:
 
That is what I'l have to try next. Even with everything now grounded I still have the problem. Part of the issue may be the balun inside the house. I have the balanced feeders coming into the basement directly beneath the shack and a short (+/- 10 feet) length of coax to a balun. I may add some length to the balanced feeder first as it will be the easiest thing to do.The impedance presented to the tuner on 14.195 is R29-j66 ohms but the R rises rapidly to about 400 ohms just above 15.2 MHz. Yes, that is fifteen point two according to the analyzer. I hear you about taking from one end and adding to the other.:laugh: I'm not about to make that mistake. Oh well later today I'll work on it some more.Hopefully I can shift the problem point outside of the band somewhere.
 
R29 -j66 is not what I suspected as being the problem. That is a very low impedance for a tuner to deal with and is made worse because of the 4:1 balun ratio.

Just lose the coax and see what happens.

Make up a 1:1 ugly balun just for the low bands. You can swap between the 4:1 and 1:1 to get reasonable loads for the tuner.
 
With an extra 8 ft of ladder line the Z presented to the tuner became R69-j143 and with just an extra 5 feet of coax it became R350-j0, a BIG difference. Using the extra lengths of BOTH types of feedlines the Z is R62-j150. Strange. In the end and for the short term I am using the extra length of balanced feeder ONLY however I replaced the AT-200 tuner with a TenTec AT-227 that has a rebuilt inductor. It seems MUCH happier now. Apparently things are picky about the exact value of inductance. The AT-200 has only positions for each band while the rebuilt inductor in the TenTec has 18 different positions available on any band. I'm happy for now and so is my rig but I still want to pursue this further. I was just short on time today.
 
I can't imagine what that setup must have cost to replace!

It was cheap to put up.

In 1944 it was just another 2M beam near Hiroshima.

Then, on August 7th of '45. it started to grow.
 
As promised. The hard part was not taking the pix but transferring them. the main PC is running updates, scans etc and is out of service at the moment. The PC I am on now does not recognize the new digital camera because it is too old (600 MHz processor LOL) and it does not take SD cards. I had to borrow one of the kids new notebooks they got for Xmas, pull the images off the SD card, transfer them to a flash drive and then transfer them from the flash drive to the old PC I am for resizing and uploading. I REALLY hope the effort is appreciated. :tongue: The design is nothing special and can be found here among many places.
http://www.pcsystems-ss.co.uk/g7lrrweb/images/pagemaster/Balun42.gif

My balun is bifilar wound using 22 turns of stranded 14 gauge THNN insulated wire. The core is a piece of plastic pipe 2 inches in diameter an about a foot long. The turns of the balun are close wound and occupy about 7 inches of the length. An SO-239 chassis connector was used for the 50 ohm connection while binding posts were used for the high impedance balanced end.


img1314sm.jpg


In the photo above you can see the green wire on the left and the white wire on the right connect together and go to the ground of the SO-239. The white wire on the left goes to both the centre of the SO-239 and one side of the balanced line.The green wire on the right goes straight to the other side of the balanced line.
img1316sm.jpg



img1317sm.jpg
 

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