[Elecraft] ½ λ dipoles

Fri Aug 5 19:09:07 EDT 2016

A 1/2 wave radiator (a.k.a. "dipole") is the shortest 'self resonant'
radiator independent of the ground. By self-resonant I mean that it has zero
reactance. 

Heinrich Hertz used a dipole in his demonstrations of electromagnetic waves
in the form of a 1/2 wave pipe bent nearly in a circle so the ends nearly
touched each other. When an identical loop nearby was excited, tiny sparks
would jump across the ends of the second loop even though there was no
mechanical connection. 

Marconi pioneered the grounded monopole which needed to be only half as
long.  

The idea of "resonance" (called "synchronicity" then) was just becoming
recognized in Marconi's time. Back then the antenna set the frequency of the
transmitter, so the antenna had to be designed to produce the desired
wavelength of signal.

Longer wavelengths were thought to produce DX. But longer wavelengths
require HUGE antennas of several hundred meters in length, even a monopole
worked against "ground". So "top loading" became very popular. 

That's where you see the multi-wire arrangements of early Ham stations. The
"antenna" was actually the wire leading up to the parallel wires strung up
high. The parallel wires provided the needed capacitance to ground to lower
the frequency (increase the wavelength). 

Since until the late 1920's most Hams clustered as close to 200 meters as
possible (the longest wavelength Hams were allowed in  the USA) in the
belief that longer wavelengths were needed for longer distances, such
antennas were very popular.

About then the "short waves" were discovered and everything changed!

73, Ron AC7AC 

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of
Charlie T, K3ICH
Sent: Friday, August 5, 2016 3:42 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] ½ λ dipoles

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)

73, Charlie k3ICH

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Phil
Wheeler
Sent: Friday, August 05, 2016 10:58 AM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] OT:. G5RV's

Alas, the poor G5RV.  Now that its been flogged to death, maybe we need a
new target -- say the Windom?

Phil W7OX

On 8/4/16 10:22 PM, Wes Stewart wrote:
> The tuner loss also depends on how it is adjusted. For example the 
> very popular high-pass Tee with three adjustable elements has an 
> infinite number of possible combinations that will effect a match on 
> the same load Z.  One of them is the lowest loss solution, all of the 
> others aren't.
>
> As I said earlier, in a letter to Dean Straw dated February 2, 1994 I 
> offered an example where the SPC tuner, then current in the handbooks, 
> could be used to match an impedance of 4.34 +j46 to 50 ohm. (I forget 
> where this came from but it was a real possibility)  I assumed Qc =
> 1000 and Ql = 300 (generous). I used Touchstone to calculate the 
> minimum loss and maximum loss solutions The best case was 1.6 dB and 
> the worst case was 7.8 dB.
>
> With lower Q components, Qc = 500, Ql =200, the losses were 2.4 to 9.5 
> dB!
>
> Wes  N7WS
>
>
>  On 8/4/2016 2:00 PM, Alan Bloom wrote:
>> > It's a pity that too many newcomers, as well
>> as many oldsters, are
>> > enamored by this piece of wire.
>>
>> The G4RV is definitely a compromise antenna.  
>> However its advantage is that is has low-enough SWR to be easily 
>> matched by most tuners on a number of bands.
>>
>> > ... the horrific losses that could be
>> incurred even
>> > with high quality tuners,
>>
>> It's true that tuner losses are the
>> manufacturers' dirty little secret. Loss is rarely specified, partly 
>> because it can be pretty bad, and partly because it is hard to 
>> measure, but also because it is not constant - it depends on the 
>> particular impedance being matched.
>>
>> One exception is the old Drake tuners.  Their Pi-L topology makes the 
>> loss almost independent of the load impedance.  If you can get it to 
>> match, you know that almost all the power is going into the feed 
>> line. For example, the
>> MN-2700 that I designed when I was at Drake was specified at 0.5 dB 
>> maximum insertion loss and I did a lot of testing and tweaking to 
>> achieve that on all bands.
>>
>> Alan N1AL

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