[Elecraft] PAR/LNR EFHW antenna

[Ron D'Eau Claire]

The EFHW with open wire line such as you describe is the classic Zepp antenna. They are a single-frequency antenna designed for the Zeppelin airships, hence their name. The radiator is exactly 1/2 wavelength long (electrically) with an electrically 1/4 wave long open wire feeder. At 1/4 wave long the feeder inverts the impedance of the antenna, transforming the very high impedance at the end of the half wave radiator into a low impedance at the transmitter. 

A real-world half wave radiator has an impedance at its end of perhaps 4,000 to 6,000 ohms so, indeed, the current the feed point is very low. The other side of the feed line simply goes to an insulator and is connected to nothing else. However there is always some small leakage current in the insulator so the balance of currents in the feed line is good. 

Of course, such an antenna, like any hunk of wire, can be made to accept power on any frequency and, like any other antenna, works best when in the clear and well above ground where the Zeppelins tended to hang out, HI! 

73, Ron AC7AC



-----Original Message-----
From: elecraft-bounces at mailman.qth.net [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of K9MA
Sent: Tuesday, January 9, 2018 5:48 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] PAR/LNR EFHW antenna

On 1/9/2018 19:25, Bill Johnson wrote:
> With a 1/2 wave a counterpoise is not used.

It needs SOME counterpoise, but not much, because the current in it is very small.  If nothing else is available, the coax shield will suffice, and causes no harm.  The small current at the end of the antenna has to have a return somewhere.  I recall old versions of the Handbook showed an EFHW fed with open wire, with nothing at all connected to the other conductor, and never could figure out how that worked.  I suppose a little common mode current in the open wire line.  Later versions, I think, showed a short wire opposite the antenna, which at least made sense to me.  The point is that the very small current at the end of the EFHW will find a return somewhere, and it really doesn't matter.

One way to tell with a QRP radio whether you need a counterpoise is to watch the SWR as your put your hand on the case of the radio or tuner. If the SWR doesn't change, you're good to go.  I don't recommend this technique at high power levels.

All of this is true ONLY for an end fed antenna which is a multiple of a half wave.

73,

Scott K9MA

--
Scott  K9MA

k9ma at sdellington.us

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