On 4 mayo, 20:39, Owen Duffy wrote:
tom wrote .net:





On 5/4/2010 3:58 PM, Owen Duffy wrote:
The technique presumably is to insert a (lossy) capacitive reactance
in series with the feedpoint, and such that the reactance decreases
with freqeuency, thus exacerbating the natural feedpoint impedance
change.

Owen

I am curious about your statement. *You say "insert a (lossy)
capacitive reactance in series". *Why would a braidless piece of coax
inserted in a tube have significantly different loss than the intact
coax of that length?

I've made matches made that way for decades that ran at full or near
full legal limit on 6m. *I'm pretty sure that any significant loss
would have shown up as dripping plastic. *The matches when taken apart
after years of use show no sign of heating.

Tom,

My use of "lossy" was to remind readers that capacitive reactance
obtained by using such a transmission line element is a relatively lossy
'capacitor'. For example. an o/c stub of RG213 for a reactance of -10
ohms at 144MHz has a resistance of about 0.1 ohms, or a Q of about 100.
That is not a huge loss, but quality capacitors achieve much higher Q
than that.

So, I don't know why one might use such a thing in a driven element,
introducing say 0.2 ohms of resistance which consumes about 0.4% of the
power if it was a R=50 feedpoint, when a similar reactance could be
obtained by a slight shortening. The purpose is probably not for
frequency compensation, it works the wrong way.

Is the loss significant, not really in this case, and it won't melt the
PE, but TL derived capacitors are relatively lower Q.

Owen- Ocultar texto de la cita -

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Could it simply be a capacitive coupling method without any matching
property ?

lu6etj wrote in
:
Could it simply be a capacitive coupling method without any matching
property ?

That depends what you mean by "without any matching property".

If the o/c stub is less than an electrical quarter wave (as it seems from
the description, it must introduce some series capacitive reactance, and
loss resistance, the longer it is, the higher the loss resistance.

It is not at all obvious why they would do what you described (and for
Tom's benefit, I am not talking about a gamma match).

Owen

On 6 mayo, 19:20, Owen Duffy wrote:
lu6etj wrote :

Could it simply be a capacitive coupling method without any matching
property ?

That depends what you mean by "without any matching property".

If the o/c stub is less than an electrical quarter wave (as it seems from
the description, it must introduce some series capacitive reactance, and
loss resistance, the longer it is, the higher the loss resistance.

It is not at all obvious why they would do what you described (and for
Tom's benefit, I am not talking about a gamma match).

Owen

Hi Owen

I mean "any matching property" in the sense that we give to a coupling
condenser in electronic circuits.

Another possibility could be that it was a method to trim the resonant
frequency easily from the center of the structure without to cut the
tube neither to make it telescopic making the element a little more
longer. I don`t know, I hoped to obtain a correct answer looking for
it in the Web.

At worse, maybe that system never existed. I only remember sometimes
heard speak about it in the eter or a mailing list :)

Thank you very much for your helping

Miguel LU6ETJ

On 8 mayo, 15:13, lu6etj wrote:
On 6 mayo, 19:20, Owen Duffy wrote:

lu6etj wrote :

Could it simply be a capacitive coupling method without any matching
property ?

That depends what you mean by "without any matching property".

If the o/c stub is less than an electrical quarter wave (as it seems from
the description, it must introduce some series capacitive reactance, and
loss resistance, the longer it is, the higher the loss resistance.

It is not at all obvious why they would do what you described (and for
Tom's benefit, I am not talking about a gamma match).

Owen

Hi Owen

I mean "any matching property" in the sense that we give to a coupling
condenser in electronic circuits.

Another possibility could be that it was a method to trim the resonant
frequency easily from the center of the structure without to cut the
tube neither to make it telescopic making the element a little more
longer. I don`t know, I hoped to obtain a correct answer looking for
it in the Web.

At worse, maybe that system never existed. I only remember sometimes
heard speak about it in the eter or a mailing list :)

Thank you very much for your helping

Miguel LU6ETJ

Another speculation...

Increasing dipole length radiation resistance increases. Could be a
method devised to take advantage of that effect using the condenser to
take the antenna to resonance again

Will it compensate the increase in the radiation resistance the higher
losses in coaxial condenser?

Miguel