[Elecraft] Small QRP antenna

[Don Wilhelm]

Fred,

You summed up antennas in general very well, and especially end fed dipoles.

I would like to add a bit to your item 7 - dealing with the matching of 
a half wave end fed to a 50 ohm line - and the counterpoise wire.

There are two common ways of matching to an end fed half wave.  One is 
by using a tuned resonant circuit with a link (or tap) to the coax.  In 
the case of a link, the short counterpoise needs to be connected to the 
'bottom' end of the resonant circuit.  If the link is isolated from the 
winding of the resonant circuit, the coax shield cannot provide that 
function, the counterpoise must be a separate wire.
If the matching network is the commonly used 9:1 unun, then the bottom 
of the high impedance side is connected to the coax shield, and the coax 
shield can serve as the counterpoise wire.  That is why one needs to use 
a short length of coax with those antennas and not connect directly to 
the rig.

Most commercial end fed matching networks can use the coax shield and no 
counterpoise wire is necessary, but you must use a short length of coax.

73,
Don W3FPR

On 12/29/2016 4:49 PM, Fred Jensen wrote:

> 7.  Feeding a half-wave at the end would seem to require that there be
> "something" for the other side of the circuit so current can flow.
> Practically, because the impedance is very high, the current is very low
> and the "something" doesn't have to be very much.  In fact, if the
> primary of the matching transformer is fed with a couple feet of coax,
> the shield will provide that "something."  So will the wiring around you
> [headphone cable, power cable, key cable], and you if you're holding
> your radio.
>