[Elecraft] Small QRP antenna

[Fred Jensen]

I'm fairly certain that Fred meant 1/2 wave ... correct hand, wrong 
finger, happens to me all the time.

This has been going on for a long time, here's a summary and maybe it 
can rest for awhile:

1.  Elecraft ATU's are L-networks.  Theoretically, one of the 4 L-net 
configurations will match any impedance to any other impedance.  
Practically, for some attempted matches, the required values of L and/or 
C become unattainable.

2.  Any conductor you can get power into will radiate.

3.  "Random" length wires will exhibit random impedances at the end [or 
anywhere else along them].  ATU's will *generally* match those 
impedances to 50 ohms [or close] for you.

4.  Some "random" lengths are better than others for matching with an 
L-network.  They all radiate however [see #2].  41-43 ft is one, ~26 ft 
is another.  Random length conductors require a counterpoise of some sort.

5.  All half-wavelength conductors are created equal in terms of 
current/voltage distribution.  The ends are open circuits ... zero 
current/infinite voltage.  The center is a short circuit ... infinite 
current/zero voltage.  The power on the conductor is constant [ignoring 
wire resistance].  It doesn't matter where you feed it.  A full-wave 
wire is just two half-waves fed in series.

6.  Infinities aren't real of course, the ends are high voltage/small 
current = Hi-Z [several K ohms].  The center is high current/small 
voltage = Lo-Z [40-70 ohms].

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.

8.  For NEC-2 models, connecting a source to the end of the half-wave 
wire means there is no return path and it essentially tries to divide by 
zero, maxing out a floating-point value.  It's a little more complex 
than that, but that's the essence.  In such a model, you need to provide 
a return path for a very small current. On 20 meters, one of my field 
half-waves requires about 6 in. Anything longer makes no difference.

One easy way to construct a "random" length antenna is to peel the outer 
cover from a length of RG-58, separate the braid at the bottom and pull 
the center conductor through the hole.  The empty braid then becomes the 
counterpoise and you can trim it to find the "sweet spot" your ATU can 
easily match.

Hope this helps on the subject.  Happy New Year to all

Fred K6DGW
Sparks NV DM09dn
Washoe County

--Northern California Contest Club
--CU in the Cal QSO Party
--7-8 Oct 2017


On 12/29/2016 7:30 AM, Fred Moore wrote:
> With an end fed 1/4 wave you only need a counterpoise that is about .05
> wave length.  it can be the coax or another counterpoise.  Although a
> counterpoise has some effect on radiation pattern of a quarter wave
> radiator, it is minimal.   You won't see much improvement in performance
> by going above .05 wave length..  it's not worth the trouble to increase
> the counterpoise length..   Fred
>
>
> On 12/29/16 9:01 AM, mjisted wrote:
>> Looking at your text message on the elecraft board. About a half wave
>> length ...here is a question for you. I'm going to put up a half
>> wavelength of wire for the 5 mhz band. ..hyendfed can I use a quarter
>> wave length of counter point as a radiator ground plane. ..
>> The. ..G0MVP.
>>
>> mjisted at btinternet.com
>>