[TowerTalk] 20 Meter

[i4jmy@iol.it]

A 5% loss with 1500W is 75w but with amateur duty cycles the wasted 
energy (joule) it's practically a lot less.
Expecially if considering that joints are typically in open air, mostly 
connected or being part of big heat sinks like booms or anyway large 
tubings, hardly the surface themperature would raise to cause permanent 
damages unless an arc takes place, but losses (if any) stay there and 
eventually get worse with time because of oxidization.
Another point that I've well clear in my mind is that things runs 
consistently different at 1.8 than at 28 MHz and not all the 160m 
experiences scaled to 10m do replicate with identical results.
Concerning the gamma (a matching sysytem that doesn't really transform 
a dipole unbalanced and involves two reactances) it's true that the 
current in the capacitor is not more than the current in the 50 ohm 
line, but it's true also that dielectric losses add to ohmic losses of 
joints when frequency raises.
Since 1500W correspond to 5.5A @ 50 Ohm, if the overall current 
decreases from 5.5 to 5.35 A because of the combination between small 
ohmic losses and small dielectric losses, the power gone is anyway in 
the range of 0.25 dB, although nothing is practically noticeable.
Stated a boom lenght, obtained similar gain, similar pattern and BW 
with two yagi antennas of different design, one fed direct at 50 Ohm 
and the other with a lower fed point impedance, it's evident the 
matching losses (if any) can't be in the yagi that hasn't a matching 
device.

73,
Mauri I4JMY



> ---------- Initial message -----------
> 
> From    : "Tom Rauch" <w8ji@akorn.net>
> To      : <TOWERTALK@contesting.com>, "Maurizio Panicara" 
<i4jmy@iol.it>
> Cc      : 
> Date    : Mon, 5 Mar 2001 19:45:28 -0500
> Subject : Re: [TowerTalk] 20 Meter
> 
> > unproper linear loading arrangements. Back to the matching circuits,
> > they may introduce losses when some of their elements have losses,
> > tipically a gamma capacitor with a lossy dielectric @ 28 MHz is one 
of
> > the the worst cases for HF, or when the mechanical construction 
(i.e.
> 
> I'm not aware of any gamma capacitors that employ lossy 
> dielectrics, mostly because lossy systems would be self-
> eliminating at the power levels we run. Most dielectrics are 
> polyethylene, which is one of the lowest loss materials available. 
> As in a transmission line at HF, dielectric losses are not a factor.
> 
> The gamma match does not carry significant current, no matter 
> what impedance element it is matching. It does not matter if the 
> element's loop radiation resistance is  5 ohms or 45 ohms, the 
> gamma rod sees 50 ohms. 
> 
> Current in the gamma capacitor is never more than the current in 
> the 50 ohm line feeding the antenna, and voltage across the 
> gamma capacitor is the reactance times the 50 ohm transmission 
> line current...which has nothing to do with loop radiation resistance 
> of the element. 
> 
> > with an hairpin) involves a number of different parts with several
> > joints and the impedances in those points is already rather low.
> > Although nothing fails and noone notices the heath, or neither a
> > signal decrease is roughly appreciable, when discussing about 
marginal
> > gains like 0.25 or 0.5 dB (a 5% or 10 % of the power) already a 
simple
> > 0.5 Ohm series resistance in a 20 OHm radiator adds an extra .22 dB
> > loss.
> 
> While it is true the current is maximum in the hairpin, if we lost 5 
or 
> 10% of the power in a joint or a few joints in the antenna we would 
> certainly have problems. Imagine 150 watts of heat distributed in 
> perhaps two connections, and picture how hot the tiny surface area 
> of those connections would become.
> 
> My L-network matching the common point of a phased array has a 
> feed impedance of approximately 20 ohms, and it uses only a ten 
> turn #8 AWG inductor bolted to a capacitor. The losses in that L 
> network, which are substantially more than I expect from a 
> reasonable size hairpin conductor, produce less than 30 watts of 
> heat out of 1500 watts applied.
> 
> Most of the joints in a traditional hairpin are at lower current 
areas 
> of the a hairpin, at the  50 ohm point of the system. Most hairpins 
> of good design do NOT interrupt the conductor at the highest 
> current area.
> 
> It makes no sense to redesign an antenna to raise the feedpoint 
> impedance to 50 ohms and give up performance, when any small 
> losses in matching could easily be handled through simple proper 
> construction techniques.
> 
> I'm reminded of Lew McCoy's silly article a few years ago in CQ 
> Magazine about terrible loss in yagi elements, and his suggestion 
> that a #16 wire element has less loss than 1"diameter aluminum 
> tubing because the wire "has no lossy joints".
> 
> With ten ohms, we only deal with 12.5 amperes. These are not 
> small loop antennas with radiation resistances of .05 ohms and 
> hundreds of amperes of current, where pressure joints do become 
> a mensurable problem.
> 
> 
> 
> 73, Tom W8JI
> W8JI@contesting.com 
> 


--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com