|[AMRadio] Ohmspun loads|
jtml at losalamos.com
Tue May 2 19:41:33 EDT 2006
Ohmspun and Ohmweve load resistors are actually quite versitile, useful for up to a few MHz. They are made with resistive wire (like thin nichrome) wound into a fire resistant fiber which is then laid down in a pattern like a grid. Across each 'card' there is a bulk resistance, like 150 ohms. I have seen them at work going up to 10 kohms per, and as low as dozens of ohms. It used to be that they used asbestos fiber, so be careful when working on one, not to breathe the dust from the elements if they are falling apart. Also, when using it with a fan, same cautions. Now they use some sort of Dacron or other material. In 1997, I contacted 'Bert', the proprietor of Ohmweve, and had a custom 100 kW pulsed RF load built in a big cage of expanded metal mesh, 1000 ohms, for 2800 KHz. I used it as a dummy load for a custom RF amplifier which I built for work. The whole thing cost less than $2000 I believe, and it was truly amazing, had a good input match (very low stray capacitance or
inductance associated with the series/parallel layout of the 'cards' in it). They transfer heat to ambient very well, being basically a heater. We use about a thousand of these Ohmweve cards where I work as series resistors on the output of big capacitor banks for the particle factory.
They are in Niantic, CT, and I suspect are using some of the same people and equipment since 1944. Bert was not a youngster in 1997. They don't have a website anymore, but a phone # can be found with Google. They make excellent BC rig loads, and were found in many of the tops of them years ago. The Globar and wirewound resistors that Gates used also worked.
From: "Todd, KA1KAQ" ka1kaq at gmail.com
Date: Tue, 2 May 2006 11:47:43 -0600
To: jtml at vla.com
Subject: Re: [AMRadio] hot water heater element dummy loads
> On 5/2/06, John Lyles <jtml at losalamos.com> wrote:
> > I gave a AM station $50 for a semi-burnt 7.5 kW Ohmspun load they used to> use for 5 kW, that was made in a cast aluminum frame. It had the resistive> wire 'toaster elements'.
> Strange that you mentioned this, John. I almost posted about these
> wire elements earlier, mainly out of curiousity. The RA-1000 I picked
> up last September came with one of these for a dummy load. One mesh
> sheet or element, maybe 2-3 feet long by 1 foot or so wide. Surrounded
> by a metal frame with large porcelain stand off insulators on the
> corners for mounting. The engineer (Bob, also on this list) told me
> that this is what they used for a dummy load and I was amazed. Not
> only had I never seen one before, it's difficult to comprehend
> something like that handling a full KW carrier!
> I take it they aren't as common as the non-inductive resistors that
> snap into those large clips?
> ~ Todd, KA1KAQ
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