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Saturday, January 18, 2020

The End Fed Half Wave Antenna and EFHW Tuners



In the SPRAT 179 (Summer 2019) article describing their Peregrino (Pilgrim) transceiver,  Joan EA3FXF and Eduardo EA3GHS recommend the use of an End Fed Half Wave (EFHW) antenna. Their circuit incorporates an EFHW tuner and an SWR indicator.  As I planned my trip to the Dominican Republic with a uBITX, I had this antenna system idea in mind.  I was attracted by the possibility of avoiding having to carry coax with me.  And it seemed that an EFHW antenna would be easier to get up in the air than a coax-fed dipole. 

When searching for schematics for EFHW tuners I came across the QRPguys tuner kits.   

https://qrpguys.com/end-fed-half-wave-sota-antenna-tuner

This looked like just what I needed, so I ordered one.  But I placed my order kind of late, and I started to worry that I might not get the tuner kit in time.  So I decided to homebrew my own (just in case!) 



As it turned out, QRPguys got the kit to me in plenty of time. It went together very quickly and is a really useful piece of gear.  My homebrew version works fine, but I prefer the QRPguys device. 

You can check out the manual here: 

https://qrpguys.com/wp-content/uploads/2019/09/efhw_40m_tuner_assy_090119.pdf

The circuits are interesting.  The EFHW antennas present an impedance not of 50-70 ohms, but of 3000-5000 ohms.   The Peregrino and the the QRPguys circuits use a matching transformer to change the high impedance to 50 ohms.  In both circuits polivaricon capacitors are used to tune for resonance. The QRPguys circuit uses an N7VE LED absorption bridge -- I found it very satisfying to put the circuit into "tune" mode and then just adjust the capacitor until the LED went out.  That means the antenna system is presenting 50 ohms to the transmitter. 

SOTA beams has a good explanation of the EFHW antenna here: http://sotabeams.co.uk/efhw/
I did use a counterpoise. 




3 comments:

  1. Show us what you made yourselve please ?

    ReplyDelete
    Replies
    1. There is a picture of it in the blog post. Same circuit. 73 Bill

      Delete
  2. Hi Bill - speaking of ordering late for an event....I ordered one for the Field Day Furlough 40 Get on the Air Extravaganza - it arrived the Monday after. :(. I'm taking it ant the F40 to the Eastern Shore this weekend. I'm really interested in trying it out. Can your recommend a good description of how the the N7VE LED absorption bridge works. I'm puzzling through the circuit trying to understand how it works.

    Dean
    KK4DAS

    ReplyDelete

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