EMRFD Message Archive 12499

Message Date From Subject
12499 2016-03-12 04:35:48 Ken Chase RF chokes

Hi All

I am working on a small signal amp. The design calls for a RF choke in the collector of a value of 120 uH at 9 MHZ. The amp is going to be for 7 MHZ, so do I need to scale this RF choke for 7 MHZ?

If so then 120 uH @ 9 MHZ on FT37-43 is 6786 ohms. Then for 7 MHZ this 6786 ohms on FT37-43 is 154 uH. Is my calculations correct?

TIA

73

Ken VA3ABN

12500 2016-03-12 09:45:21 Jim Strohm Re: RF chokes

Ken,
Was the original amp designed with the aid of modern modeling tools, or just something that somebody threw together that worked? In other words ... do the transistor's input and output impedances support the 6.8Kohm choke value, or could it conceivably be something lower?

I'm wondering whether maybe a 100 uH choke might have worked, but the designer only had a 120 uH in his junkbox. Hence the published value.

If the choke really ought to be 6.8Kohm at 9 MHz, then your calculations look correct -- or close enough, anyway.

73
Jim N6OTQ

12501 2016-03-12 17:30:06 Ken Chase Re: RF chokes

Hi Jim

The circuit is Wes' Hycas amp, so I assume it is correct. As it utrns out the reactnace between the two frequencies is not much, so I went in between turns wise.

73 Ken VA3ABN

12502 2016-03-13 10:01:23 Jim Strohm Re: RF chokes

Sounds good, Ken.

As most of us should know by now, some component values aren't as critical as we make them out to be.

73
Jim N6OTQ

12499	2016-03-12 04:35:48	Ken Chase	RF chokes
Message	Date	From	Subject
Hi All I am working on a small signal amp. The design calls for a RF choke in the collector of a value of 120 uH at 9 MHZ. The amp is going to be for 7 MHZ, so do I need to scale this RF choke for 7 MHZ? If so then 120 uH @ 9 MHZ on FT37-43 is 6786 ohms. Then for 7 MHZ this 6786 ohms on FT37-43 is 154 uH. Is my calculations correct? TIA 73 Ken VA3ABN
12500	2016-03-12 09:45:21	Jim Strohm	Re: RF chokes
Ken, Was the original amp designed with the aid of modern modeling tools, or just something that somebody threw together that worked? In other words ... do the transistor's input and output impedances support the 6.8Kohm choke value, or could it conceivably be something lower? I'm wondering whether maybe a 100 uH choke might have worked, but the designer only had a 120 uH in his junkbox. Hence the published value. If the choke really ought to be 6.8Kohm at 9 MHz, then your calculations look correct -- or close enough, anyway. 73 Jim N6OTQ
12501	2016-03-12 17:30:06	Ken Chase	Re: RF chokes
Hi Jim The circuit is Wes' Hycas amp, so I assume it is correct. As it utrns out the reactnace between the two frequencies is not much, so I went in between turns wise. 73 Ken VA3ABN
12502	2016-03-13 10:01:23	Jim Strohm	Re: RF chokes
Sounds good, Ken. As most of us should know by now, some component values aren't as critical as we make them out to be. 73 Jim N6OTQ