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Homebrew A great first timer project

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ka3jjz

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#1
We've all read the horror stories of how someone forgot to disconnect their antenna and the radio got zapped - or perhaps how static electricity fried some portable's front end

Let's face it; there are going to be times when you will forget to disconnect the antenna (and ground it away from the home, of course).

There is also the problem of too much antenna. Little portables like some of the Kaito models simply aren't designed to handle the amount of RF a 50 or 75 foot antenna can deliver, particularly on the East Coast of the US. It would be nice to be able to add a little attentuation from time to time when it's needed.

The JPEG below is a schematic of a simple box anyone can build. The parts can be had at any decent hamfest, and it's a great first timer project.

It might be necessary to replace the diodes with something like a small light bulb if you are using a good sized antenna in an area like northern NJ or NYC (my home area) where AM signals are very strong. It's just possible that the diodes could begin to rectify and cause noise and other issues.

In fact this is also a good addition for those using the Carpet Loop antenna- it can be put right into the switchbox with very little effort.

The JPEG is below...it comes from an older ICF2010 page, by the way. Adjust the input plug to whatever is needed - it's not all that critical

Mike
 

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#2
Mike, I have a set up similar, but lacking the 1k- 10k potentiometer. I noticed that my signal with the set up I use doesn't seem as strong, no problem, rather be safe than sorry. My question is would using this set up you posted with the potentiometer compensate for my loss ?
 

ka3jjz

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#3
I rather doubt it. While the pot would be guilty of having some insertion loss, it's main function is as an external gain control, something many portables don't have. How much insertion loss there would be would be kinda difficult to quantify without measurements.

If you're not experiencing overloading, you could leave it out. But so many portables overload so easily with, say a 50 foot wire that it's a simple solution...Mike
 

Phil_KD0SCJ

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#5
Two questions...

1. Can the potentiometer be wirewound? I happen to have on sitting around...

2. One would put this in a case... does it need to be a metal case or would a plastic one do?

Thank you!
 

ka3jjz

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#6
Hmmm...the last time I saw a wirewound pot, it was in an old TV I tore apart for the parts; you won't hurt anything to try it, though.

I would defintely put this in a plastic case just because it's easier to work with...just my 2 pfennig....Mike
 

Phil_KD0SCJ

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#7
I found a more suitable pot with a plastic resistive element... Anyway, I ran into an interesting issue with the diodes. I connected them and a nearby AM broadcast tower completely took ownership of my radio, well past 15MHz. I cut them out and it was all good. I am thinking about substituting gas discharge elements. Any thoughts on that?
 

ka3jjz

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#8
You found the exact issue that I mentioned in the first message of this thread.

Yep, a gas discharge element will do just fine. Anything like this that will blow before your radio does is worth looking at

Mike
 
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