[Amps] Building an Amp 101

[Fuqua, Bill L]

   Most relays are not constructed of materials suitable for RF. Plastic rather than ceramic is an example. They use a leaf spring material that can handle current just fine at 60HZ or DC but it has a higher resistance than copper. And since the current doesn't flow thru the entire cross section, only on the few microns of skin depth the RF resistance is much higher. To get the gap needed you would need open frame relays, which take up a lot of space. I have had one surplus relay bandswitched amplifier which used the open frame relays that used to be used as ladder line antennas switching relays. But it was a very large box with a pair of 813s in it. 

73
Bill wa4lav

________________________________________
From: Amps [amps-bounces at contesting.com] on behalf of Al Kozakiewicz [akozak at hourglass.com]
Sent: Sunday, January 19, 2014 10:43 AM
To: MU 4CX250B
Cc: amps at contesting.com
Subject: Re: [Amps] Building an Amp 101

To make it clear, I've never tried this so I'm not speaking from personal experience.

But it seems logical to me that if a vacuum relay is required for the voltages and duty involved, then a bandswitch is out of the question as well, isn't it?  My thought is that, putting aside questions around stray-L that might arise from the insertion of a relay, it's easier to find a relay with sufficient contact spacing than it is to find a bandswitch with a large enough diameter to maintain the same spacing between contact elements.

I guess these sorts of problems is why it's called engineering and not science!

Al
AB2ZY

-----Original Message-----
From: MU 4CX250B [mailto:4cx250b at miamioh.edu]
Sent: Sunday, January 19, 2014 9:32 AM
To: Al Kozakiewicz
Cc: amps at contesting.com
Subject: Re: [Amps] Building an Amp 101

Relays are certainly a possibility, Al, but for a nine band amplifier things get a bit complicated. My latest project ( 90% completed) is a duo - band 80m/160m amplifier with a pi- L tank circuit, and for that I am using two DPDT vacuum relays, each of which cost over $100.
Scaling that up to a nine band amplifier doesn't seem very practical or cost-effective.
73,
Jim w8zr
Sent from my iPhone

> On Jan 18, 2014, at 8:52 AM, Al Kozakiewicz <akozak at hourglass.com> wrote:
>
> Why not use relays appropriate for the switching duty?  Then the bandswitch can be just about anything or even remotely controlled.  Plus you can locate the switch where it's convenient to operate instead of being constrained by the necessity to place it close to the RF components.
>
> Al
> AB2ZY
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