[Elecraft] ½ λ dipoles

[Walter Underwood]

Some of the early “clothesline” antennas were a large capacity hat on a vertical. If the antenna has one vertical wire connected to all of the top wires, it is probably a capacity-loaded vertical. This Wikimedia image shows a top-loaded vertical.

https://commons.wikimedia.org/wiki/File:Amateur_radio_T_antenna_1912.png <https://commons.wikimedia.org/wiki/File:Amateur_radio_T_antenna_1912.png>

With longwave communication, a resonant antenna was not practical for most hams, whether horizontal or vertical. I certainly don’t have room for a half-wave for the 600 meter band.

wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)

> On Aug 5, 2016, at 4:53 PM, Don Wilhelm <donwilh at embarqmail.com> wrote:
> 
> 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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