[Elecraft] ½ λ dipoles

Fri Aug 5 20:23:11 EDT 2016

What Don says is quite right for the 1930's, 40's and onward. I was
describing the early antennas of the teens an 20's. They have but one wire
feed with several parallel wires the last few feet to connect to the
horizontal wires. 

Once Hams moved quickly from 200 meters to 80, 40 and even the rarified high
frequency of 20 meters the antennas changed accordingly. 

Multi-wire folded dipoles were, as Don says, an easy way to match 600 ohm
open wire lines (although few Hams cared about SWR) and had the side
advantage of broadening the frequency response. 

73, Ron AC7AC

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Don
Wilhelm
Sent: Friday, August 5, 2016 4:53 PM
To: Charlie T, K3ICH; elecraft at mailman.qth.net
Subject: Re: [Elecraft] ½ λ dipoles

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)
>
>

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