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Author Topic: source of variable capacitor  (Read 2626 times)
WS4T
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Posts: 182




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« on: November 11, 2010, 11:37:07 AM »

I want to build an inverted-L for the 80m band. The ARRL Antenna Book (21st edition) has an antenna for 160m on page 6-42. There's a variable capacitor in series to tune out the inductive component.

My question: What sort of creature is this variable capacitor? I've been doing some searching. The best I can come up with is a bunch of Russian surplus capacitors like this: http://www.nd2x.net/ur4ll.html (search for "variable cap")

Is there an easier solution? My local electronics supplier doesn't seem to have much in the way of variable capacitors.

Tnx!
Gary, WS4T (ES1WST)
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W7SMJ
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Posts: 120




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« Reply #1 on: November 11, 2010, 12:27:29 PM »

What value is the variable cap in the circuit?  How much power are you planning to run? 

There are many sources for variable capacitors, just google it.  If you can't find any surplus, MFJ stocks capacitors they use in their tuners...
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K7KBN
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« Reply #2 on: November 11, 2010, 12:29:32 PM »

You didn't say anything about what size capacitor you need, but a few seconds' search at RFParts.com brought up this page: http://www.rfparts.com/caps_variabletuning.html
which may or may not have the correct component.  If not, search further on the RFParts website and maybe a few more.

73
Pat K7KBN
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73
Pat K7KBN
CWO4 USNR Ret.
AD4U
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« Reply #3 on: November 11, 2010, 12:45:18 PM »

If you don't want to pay an arm and a leg for new capactors, check out the flea markets at hamfests.  Most variable capacitors at hamfests do not sell any more.  Consequently they usually can be had for bottom dollar.

I guess this is because most hams either prefer to buy a factory built matching unit or they do not know what is inside the mystery matching box and don't care to learn.

Kudos to you for wanting to build a matching network.

Dick  AD4U
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K8AC
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Posts: 1465




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« Reply #4 on: November 11, 2010, 02:28:18 PM »

I use a series cap with my inverted L on 160M - can't say what the adjusted value is with the 160 foot antenna, but I used a surplus cap of maybe 400pF with 1/8 inch plate spacing.  The voltage rating you want will depend on how much power you run.  I run 1500W to mine with no problem.  MFJ sells new caps that would do, and you'll find good ones at www.surplussales.com - just search for variable capacitors and they're at the top of the page.  There are also many on eBay right now.

73, Floyd - K8AC
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K4DPK
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« Reply #5 on: November 11, 2010, 03:00:40 PM »

Hi Gary...

You might consider doing it this way....

Get a receiver type double or triple ganged capacitor, then connect it and adjust it at a low power level.  When it is at optimum, remove it and measure the capacitance without having disturbed the setting.  Then, depending on the power level anticipated, construct and weatherproof a bank of fixed capacitors of the appropriate (same) value and install that.

Phil C. Sr.
k4dpk
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KZ1X
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« Reply #6 on: November 11, 2010, 05:02:40 PM »

http://www.mgs4u.com

http://www.mgs4u.com/RF-Microwave/air-variable-capacitor.htm
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WS4T
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Posts: 182




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« Reply #7 on: November 13, 2010, 11:29:15 PM »

Hey Everybody,

Thanks! Exactly what I was looking for.

73,
Gary
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AC5UP
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Posts: 3835




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« Reply #8 on: November 14, 2010, 08:53:18 AM »

On a vaguely related note...

Many moon ago a local radio station moved their AM transmitter site and took the big money option. New antenna, blockhouse, transmitter, took very little from the old site. Friend of mine was the CE at the time. Called me on the phone and told me where the old site was located, back door unlocked, I was invited to haul out anything that tickled my fancy.

Ten kay-dub Western Electric ether heater of 60's vintage with tons of spare tubes from both the TX and studio equipment.

I hauled out something like six or seven car loads of junque and some of the parts were very interesting. Ceramic variable link coupling coils (coil within a coil, looks like a gimbal mount, sometimes called a Goniometer) big, big, big oil filled HV caps, w-i-d-e spaced variable caps and the mother of all HV variable capacitors.... Visualize a 200 pf beastie slightly larger than an R-390 (I kid you not), reduction gear drive, white ceramic tuning shaft about 18" long with doughnut sized ceramic insulators as feet. It was a very impressive piece.

I hauled it down to Dallas for the First Saturday Sale as a "make offer" item and no one bought it. Didn't want to haul it back home so I gave it to a friend that ran a surplus house I'd frequented for years. The deal was that if he could find a good home for it please do, otherwise I'd haul it off next month if he wanted his floor space back. If he could turn a fast buck on it that was very OK 'cuz when I give something away it's no longer mine. It's your beast now, your call, have lunch on me.

Did I mention the capacitor weighed about 60 pounds and was 100% solid cast brass, both rotor and stator?

Never crossed my mind while I had it, but after a few weeks he took it to a metals recycling place. Never did hear how much he got for it but there was no complaint about how things worked out..................  Grin
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WB0GOA
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« Reply #9 on: December 04, 2010, 08:49:18 AM »

If you are using the matching network at the base of your antenna you can used RG 58, RG 59, or RG 6. RG 58 has about 30pf and 59 and 6 around 20pf. If you think you need around 300pf take 22' of RG 6 and start cutting off the end till you have your match. Weatherproof the end, wind it up in a small coil to save space. This type of cable is hanging around everywhere. I used this on 160/80 1/2 and 3/8 wave Ls to tune inductive reactance out. Cheap high voltage capacitor, but not variable. You can add a switch with two of these caps to tune the whole band.
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WB6BYU
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« Reply #10 on: December 04, 2010, 10:40:40 AM »

I have a box of old postage-stamp mica capacitors (probably 1950's vintage) that I use in series/parallel as
needed.  I modeled my current inverted L and figured I needed around 470pf.  That worked fine, using one of
the smaller ones, but then I replaced it with two of the larger 820pf units in series to handle higher power.
(Not that I'm likely to stress it at 100 watts.)

With 470pf, 220pf, 100pf, and 47pf you can combine them in various ways by experiment to figure out how
much capacitance you actually need if you don't have a variable capacitor handy.  Even if you are using a
variable capacitor, you can then replace the experimental unit with a more permanent solution once you know
how much you need.  (Or just measure the input impedance at the feedpoint with an SWR analyzer and
calculate the capacitance required to equal the inductive reactance.)
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