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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
Feb 8th 2020, 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..
Aug 7th 2021, 18:55

richardpavelko

Joined: Aug 4th 2021, 19:11
Total Topics: 0
Total Posts: 0
I know Im late to the party here but wanted to explain why this antenna wasnt working.
He was using a half-wave wire with a 9:1 matcher. A half-wave wire will have an impedence of 2k to 3k ohms. A 9:1 matcher is looking forvan impedance of about 450 ohms. Try a length NOT a half-wave on the bands you want to work.
Feb 16th, 22:31

K0WUQ

Joined: Dec 3rd 2012, 11:13
Total Topics: 0
Total Posts: 0
I too am very late, and hope that the party is not already over.

First, I think EQU is really good to have in your call sign. It reminds me of my career in computer programming, and is evocative of many kinds of equality, equanimity, etc. This could be used very creatively on your QSL cards, for example.

But, down to business. Here's what I think is the best way to use your half wavelength of wire, especially since you're so good at coil winding:

Make about 6 or 7 hokey lightweight wooden spacers with two holes about 8-10 inches apart, drilled just big enough to pass the wire through. Make a couple of extra holes near the center of one spacer. This one is the Quarter Wave Stretcher. Thread one end of all of them onto your wire close to the near end, with the Quarter Wave Stretcher first, so it's farthest 'out'.

Run the wire out on the ground, keeping it as smooth and straight as possible, then bend it back on itself so you're bringing the 'far end' back in toward the 'near end'. Thread the end you've brought back through the unused holes in the stack of spacers. Temporarily tie the ends together with a wire tie and walk the stack of spacers back out, dropping off each one so you have roughly equal spaces along the U-shaped quarter wave length. The Quarter Wave Stretcher should be pulled tight right at the end so there are two 90-degree bends in the wire as it spans across the spacer. Take a shorter length of wire (maybe 4-6 ft) and thread it tight through the two center holes in that end spacer so the ends reach farther out beyond the spacer - these have no connection to your antenna wire, they're just for tying that end to a pole, shed, pickup truck or something. Space out the remaining spacers the best you can with the one nearest the transmitter just a foot or two from the near ends of the wire, then go down the whole length and put a dab of silicone glue or something on each hole where the wire goes through. You now have a long U-shaped antenna with 150 ohms impedance.

Now you'll need an ordinary plastic pill bottle and a bunch of insulated hookup wire - for a QRP rig, ordinary solid no. 22 wire will do nicely. Wind 14 turns of this wire snugly around the bottle so there's some uncovered bottle at each end, bring the two dangling ends across each other and twist together 2 or 3 turns to hold it - BUT no too tightly! You have just finished the Secondary of your matching transformer. Now take another piece of insulated wire, preferably a contrasting color, and do the same thing for exactly 8 turns, starting about 3 turns in from the end of your Secondary coil, and forcing the new wire in between adjoining turns of the Secondary you just made. Heaven help you if you're trying to do this with 20 ft of wire! (8 turns around a 1-inch diam pill bottle will take about 25 inches, plus a little for 'end leads'.) The two coils tightly wound together like this is called a 'bifilar' winding, something you'll need to remember for the Pop Quiz later. Secure it like you did the Secondary, by twisting the two ends together. Add some silicone glue to each end of the Secondary to keep the whole thing together.

Now stretch your antenna out as high as you can manage. Striving to keep the two halves of the wire PERFECTLY parallel is totally wasted effort. One end of the antenna wire goes directly to ground, the other end is grounded through the Secondary of your transformer. The 8-Turn Primary of your transformer goes across the Antenna terminals (probably Antenna and Ground) on your transmitter. What will scare the heck out of you is that the primary looks like it would be a dead short on the transmitter output - but IT IS NOT.

Which end of each coil goes to ground makes NO DIFFERENCE AT ALL. If you want to put a simple Send/Receive switch at the connection to the transmitter, you'll find that this antenna works great as a receiver antenna as well.

The transformer matches your 50 ohm output to the 150 ohm antenna. This is an impedance ratio of 3. The transformer is virtually perfect, because with your calculator you will find that 14/8 is deadly close to the square root of 3.

Larry K0WUQ
May 11th, 18:33

k1qi

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
All sorts of late activity!

The basic issue here is trying to use a 9:1 unun with an end-fed half wave antenna. That kind of antenna has a 2500 ohm resistive impedance at resonance. A 49:1 unun is needed to transform that down to approximately 50 ohms. The 9:! unun will result in a mismatch at the antenna, high SWR on the coax, RF around the transmitter, and poor loading without a tuner.

The ARRL currently sells a half wave end-fed antenna in the shop - you can look at the instructions there. You can also search for EFHW antenna and find a wealth of information. Another good search is: 49:1 unun.

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