Hi All, there is one gotcha using the strap or wire and that is where you
route it through a metallic panel/ chassis etc to get rf from one side of
the panel to the other.
The ground return doesn't want to penetrate the panel and can take a
really circuitous route from one side to the other. The result is added
inductance. Using a piece of coax eliminates that uncertainty.
73,
Gerald K5GW
In a message dated 12/6/2013 11:40:04 A.M. Pacific Standard Time,
4cx250b@miamioh.edu writes:
I have always thought that for lengths much less than a wavelength,
the impedance looking into a transmission line is dominated by the
impedance of the load connected to the end of the line. The
characteristic impedance of the transmission line is pretty much
irrelevant until its length approaches a fraction of a wavelength.
Thus, for short straps at HF frequencies, the impedance of the strap,
length of coax, or whatever, doesn't really matter. I believe this is
consistent with Carl's opinion.
I have heard audiophiles worry that the "impedance mismatch" of their
50 ohm phono cables with the 1Kohm input impedance of their preamps
introduces distortion. As they say, a little knowledge is a dangerous
thing.
73,
Jim W8ZR
Sent from my iPhone
> On Dec 6, 2013, at 11:54 AM, Jim Thomson <jim.thom@telus.net> wrote:
>
> Date: Fri, 6 Dec 2013 10:21:59 -0500
> From: "Carl" <km1h@jeremy.mv.com>
> To: "Jim Thomson" <jim.thom@telus.net>, <amps@contesting.com>
> Subject: Re: [Amps] PARALLEL CAPS IN OUTPUT
>
>>
>> ## For folks with a PI net, make sure you use a piece of 50 ohm coax
>> between the load cap and the TR relay.
>> And with shield bonded to chassis at BOTH ends. If wire or cu strap is
>> used instead...then u end up with a huge Z bump.
>
>
> ** The coax isnt long enough to act as a transmission line at HF....in
most
> amps anyway.
>
> #### OK, do it ur way. Use a chunk of bare wire, strap etc..and watch
the
> PI net load cap values change asap. Ditto with coax...and only 1 end
of shield bonded to
> chassis.
>
> ### strap, wire, etc is NOT 50 ohms, not even close. Try it urself.
>
>
>
>> That alone will put the load cap at something higher than a 50 ohm
>> point... increasing the peak V. On 10m band, the
>> stray L of a piece of wire instead of coax will make the PI now
appear
>> as a PI-L.... putting the load cap at a much
>> higher Z point. same deal peak V increases. Now with a wide open
on
>> the ant, coax cable etc, the load cap
>> will weld shut.
>
> ** All the coax does is act as a shielded cable with some value of C.
> At 10M even 1/10 wave of cable is 1.6' so it is invisible as coax.
>
> ##### I don’t believe it for a second. Of course, I have tried it
both ways.
> Replacing the junk typ used from load cap to TR relay will solve..and
has
> solved all sorts of issues.
>
> Later....... Jim VE7RF
>
>
>
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