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End Fed QRP Antenna

Apr 21st 2017, 18:35

kd9equ

Joined: Sep 30th 2015, 16:52
Total Topics: 0
Total Posts: 0
I followed instructions on youtube (9:1 unun video) to create an end-fed QRP antenna. Basically a 20 meter half wave 14 gauge wire attached to a 9:1 unun I wound myself on a T130-2 torroid. I tested the 9:1 I created using 450 ohms of resistance connected to the wire end and ground. At that point the ohms were roughly in the 50-60 range. So I went about soldering a 239 end on and installed in a box. Upon putting the wire up in a tree about 25 feet, I am getting high ohms (around 100) and high swr on 20 meters (as in 15:1 or higher). I also noticed that when touching the radio the swr changes (usually lower by 1 or 2 units). The receive noise reduces as well. I suspected a ground issue (which both the videos and docs online say I don't need a counterpoise for this design) . Anyway, I tried a counterpoise but this did not change my SWR any more than me touching the radio. I also tried to put both a ground wire from the radio AND a counterpoise in... Still no luck. I have also played around with the wire length of both the long wire side and counterpoise... I have checked the 9:1 is still good and wired correctly... I have spent a bunch of time researching and haven't found much of anything to try. I am trying to learn along with this process so any ideas / help would be appreciated.
Apr 21st 2017, 20:11

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
It sounds like you have some sort of basic wiring problem. I'd suggest testing the coax, perhaps with a dummy load and separate antenna.

It can be very frustrating to figure out what is wrong. Sometimes the best thing to do is to set it aside and build something else, then get back to it later. Having someone check your work isn't always practical.

Zack W1VT
ARRL Senior Lab Engineer
May 26th 2018, 13:27

W3HMS

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
what 475 KHz articles have appeared in QST
Feb 8th, 11:27

WD3D

Joined: Mar 1st 2011, 09:28
Total Topics: 0
Total Posts: 0
Maybe the problem is - that you did not take into consideration the dielectric constinant of the wire used for the half wavelength dipole.

Or the fact that an end fed antenna is a poor radiator.

It's bandwidth is usually just a couple of kilocycles plus or minus 15, which is not a very good antenna for someone running 5 watts or less. Antenna tuners or trickers as I like to call them are very inefficient - throws away power trying to trick the radio into believing that the antenna is resonant..

And most people so lazy to use a end fed antenna is also too lazy to get that antenna 50' - 60' off the ground where it might do some good. More than likely if you can't find a match it is because of one of those reasons, or because you have it too close to a building, vehicles sitting beneath it or trees and you are getting a reflection off those objects..

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