[Elecraft] ½ λ dipoles

[Don Wilhelm]

Charlie,

A bit of history ---

Most of those ham antennas that used parallel wires were folded dipole 
antennas - yes they were mostly 1/2 wavelength long.  The feedpoint 
impedance for that antenna is 300 ohms.  Add a 3rd wire or a 4th and the 
impedance increases.  So to my mind, that was an attempt to match the 
feedline to the antenna which in early days was open wire line which for 
normal spacing has a characteristic impedance near 600 ohms.
By the time I became a ham, TV twinlead was common with a characteristic 
impedance of 300 ohms.  Many ham antennas were created using that 
twinlead.  A folded dipole was made from the twinlead and fed in the 
center with additional twinlead serving as the feedline.

With the migration to coax feedlines, those older techniques have faded 
from memory, but those antenna *did* work just fine although many hams 
did not really understand why.

At that time we had PA tank circuits with swinging link coils and could 
match most any impedance.  The tuning sequence was to start with the 
link lightly coupled to the PA inductor and then to "dip the plate" to 
resonance - then slowly increase the coupling between the PA inductor 
and the antenna link to increase the PA current.  That was done in an 
iterative manner until the plate current was at the desired point.
That process could match most any load that the antenna and feedline 
might present to the transmitter.

Then came television.  Many ham transmitters were interfering with TV 
reception, so transmitters became shielded devices, and the shift to 
coax rather than open transmitters with the older parallel feedline 
connection direct to the antenna slowly became a product of the past.  
Swinging links and plug in coils inside a shielded enclosure were 
possible, but a PITA.
So the advent of the Pi-Network in ham transmitters was born.  It 
allowed band switching and could match a reasonable range of antenna 
impedance.  The shielded coax feedlines provided the chassis shield to 
be extended all the way to the antenna feedpoint (or so the story goes, 
but that is not entirely true).

The bottom line of what I am trying to communicate is that much of ham 
radio antennas, transmission lines and transmitter construction changed 
drastically in the 1950s with the advent of television and that was done 
primarily to reduce ham interference to TV viewing (TVI).
As an example of that effort, my first novice transmitter which I built 
from a design in a 1955 ARRL Handbook was in a completely shielded 
enclosure and used shielded wiring throughout with bypass capacitors at 
each end of the shield wire.  That included all the wiring, filaments 
and DC power circuits and anything else.  If you find a 1955 ARRL 
handbook it was the 75 watt transmitter with a 5763 crystal oscillator 
and 6146 final included in that book. Nostalgia urges me to again build 
that transmitter, but practical sense says that it would be 
prohibitively expensive these days and some components are no longer 
available.

73,
Don W3FPR

On 8/5/2016 6:41 PM, Charlie T, K3ICH wrote:
> I'm curious as to when the concept of a ½ λ dipole became the norm?
>
> In other words, the idea of the current distribution as exists on a dipole.
>
> Early pictures of typical ham antennas looked more like a set of parallel
> clothesline wires.
>
> What I gather from reading early articles,  it seemed that the more wire you
> had in the air, the better it would "capture" (and radiate) the signals.
>
> Feel free to reply directly if you don't want to clutter the forum.
>
> (k3ich at arrl dot net)
>
>