End fed half wave tuner - a handy tool for fieldday & /P
Thank you to OZ8NJ for making me wiser in the ways of matching!.
I have worked alot of stations /P with the broad band 1:49 transformer
used by many people for feeding half waves. But the losses are not
something to be proud of with that solution, it works yes, but in the
10m band, the losses approach 50% depending on your setup.
So i wanted something better! (dont we all :-)
The solution i settled on is the L-network used in many auto tuners
like the wellknown LDG & MFJ products.
Only drawback is that you have to know the final impedance of your
antenna system to be able to match it with the L-match. This filter
design is also only useable on the single frequency you design it for.
The setup i use is vertical half wave wires mounted on a glassfiber pole
wrapped around the mast a couple of times or secured with tiewraps.
The antenna feed is mounted close to the ground in most scenarios
and that gives my system an impedance in the vicinity of 2000 ohms.
Typical L-network
Both types are for matching a high impedance antenna to your 50ohm radio.
The losses in the L-network compared to the autotransformer are alot better
but the circuit can only match 1 specific load at 1 specific frequency, and i did not
want to have 6 different boxes for 6 bands... i had to come up with something.
I decided to make a all-in-one-box solution with 6 switchable inductors
and i started calculating the 6 different filterdesigns needed for
80-40-20-17-15-10m.
Here are the values for my chosen setup. all filters are calculated for
the phone section of each band. you can find a calculator in the menu
to the left, for designing filters to suit your own needs.
I found an old 10-200pF air variable capacitor which will do nicely.
Then i made the 6 inductors using T94-6 cores for all but 80m, that
one goes on a T94-2.
I found a plastic box to put it all in. and i also had an old 2 pole switch
with 6 positions, just what i needed to switch between the 6 inductors.
My circuit is the lowpass type shown at the top of this page.
Only one thing missing and thats a way to indicate when the antenna
is tuned. One way of doing it would be to have a small swr meter inline
at the antenna but naaaa.. didnt come this far to just add another box :-)
So i found this nice little circuit on the web, and thought why not give it a go.
I mounted the small T50-43 core right on top of the coax input connector
and made a small veoboard with the remaining circuit.
Theres a piece of normal copper clad PCB running the width of the box
to be used as internal connections groundplane.
UPDATE ! Jun. 18th 2014
After testing the design, the SWR circuit had a very wide null at min. REF so
i had to make some changes to the bridge circuit. I now use matched
germanium diodes and 5 more windings on the toroid. There was
also a rather big issue with the balancing circuit.
If you decide to use this circuit, please build the revised version shown below.
a couple more images of the revised circuit
After redesign of the SWR bridge i decided to use an analog meter to indicate
the SWR instead of the LED's. I found a nice little 100uA meter at a HAM meet
and that fit nicely onto the lid. Theres a better "feel" when adjusting this way.
UPDATE ! Jun. 19th 2014
After the success with my end fed tuner i decided to rework an old
Nissei meter i had, which was made for 26-30Mhz into a true HF model.
So i fabricated another bridge circuit and rebuild the original one,
it works like a charm. I can honestly say that this SWR bridge is
a very reasonable design, and easy to reproduce.
the rebuild SWR bridge, now fully HF 1-30Mhz functionel :-)
UPDATE ! Jul. 22 2014
After alot of recent antenna testing, i was SOOO tired of having to switch
cables and radios, so i decided to build a switch and include my latest
swr bridge into the mix. i have designed it with 2 radio inputs and 3
antenna inputs. these can be linked and matched to my liking.
It works very well, and it's a real treat to have the SWR indication.
(sorry about the poor quality phone image)
UPDATE ! Jul. 22 2014
Yet another update haha, but i thought to myself, now that i have got this
far, why not go all the way. i have remote tuners on all of my antennas and
that means Bias-TEE on all cables. So i decided to include that in my
newest addition to the shack. I have used the circuit described on
my "Bandpass" project page.
