[Elecraft] KX2

[Don Wilhelm]

Dave,

Most of the commercial end fed halfwave antennas are designed for low 
power (100 watts and under).  So the use of the coax shield as the 
counterpoise element is not a big problem in most cases.

Add to that, the EFHW is an easy solution for portable operation - an 
easy up effective antenna.

At the home station where more than QRP power is common, that is where 
the difficulties with EFHW antennas become more problematic.  In other 
words, RF-in-the-shack at QRP levels may not be noticable, but at higher 
power levels, it can cause problems.

That being said, I can run my LNR/PAR EndFedZ for 40/20 meters at 100 
watts with no apparent problems.  I would not want to push it much higher.

73,
Don W3FPR

On 7/14/2016 8:26 PM, David Gilbert wrote:
>
> Yup ... that all makes sense.  The counterpoise as described by that 
> link is at the antenna where it should be, and it compensates for the 
> fact that the network can't produce an infinitely high impedance 
> feed.  I stand corrected.
>
> Using the coax for a "counterpoise" is a really bad idea, though. 
> There would essentially be no defined counterpoise.   There would be 
> no control at all over what kind of balance the coax provides to the 
> matching network at the end of the EFHW.  Length of the coax would 
> have an effect, and not just the requirement for some minimum length 
> ... the transmission line effect of the length would change its 
> characteristics as seen by the antenna. Proximity of the coax to 
> nearby structures would also affect what the antenna actually saw.
>
> 73,
> Dave  AB7E
>
>
>
> On 7/14/2016 4:56 PM, Don Wilhelm wrote:
>> Dave,
>>
>> All that is theoretical does not translate directly to the physical 
>> world.
>> Take a look at http://www.aa5tb.com/efha.html for more information.
>> The counterpoise does not need to be very long, but it does need to 
>> be present.
>>
>> If the coupling between the high impedance side of the matching 
>> device is connected at the "ground side" to the shield of the coax, 
>> then the coax shield can act as the counterpoise (with the attendant 
>> risk of RF-in-the-shack), but does require some minimum length of 
>> coax to be effective - the PAR EndFedZ is one example.
>>
>> 73,
>> Don W3FPR
>>
>>
>> On 7/14/2016 5:40 PM, David Gilbert wrote:
>>>
>>> I'm confused why an EFHW should need a counterpoise.  If it needs a 
>>> counterpoise it isn't actually acting like an EFHW. If it needs a 
>>> counterpoise that means there isn't enough choking impedance at the 
>>> feedpoint, and it means that the feedline is radiating with the 
>>> counterpoise acting as ... well, a counterpoise.
>>>
>>> Manufacturers state a minimum length feedline simply to have the 
>>> feedline losses help swamp out SWR variations along the line, and of 
>>> course to marginally lower the SWR.
>>>
>>
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>
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