[MRCA] [armyradios] RE: AB-1241 - DC current rating?

[J. Forster]

> Thanks, how does one rate a capacitor's power handing capability? I've
> seen WVDC ratings, but what do I look for?

Transmitting capacitors have current ratings.

Power = I^2 * R; so 50 Watts @ 50 Ohms is 1 Amp. It'll be higher because
the VSWR is >1:1. I'd look for a DC block capable of handling a few Amps
of RF.


> And John - if I put the cap between the antenna and the tuner, then
> wouldn't the tuner tune out the capacitance? Or do I still need to look
at something around 50 ohm at HF?

You want the capacitive reactance  (Xc = 1/ 2*Pi*F*C) to be very small.

For example 10,000 pF (.01 uF) = 1.6 Ohms at 10 MHZ. You need a good sized
cap, IMO.

I don't know if they still do, but ATC (American Technical Ceramics) made
this kind of capacitor.

You might have to build your own DC block with a couple of connectors, a
length of copper plumbing pipe and a capacitor, and a big soldering iron.
Hint: Hobby shops & hardware stores sell thin walled brass tubing that
would be nice as a center conductor.

Look at the formula for the impedance of an air spaced coax line to figure
out the diameters needed.

Best,

-John

====================


>
>
> 73 Eugene W2HX
>
>
> -----Original Message-----
> From: mrca-bounces at mailman.qth.net [mailto:mrca-bounces at mailman.qth.net]
> On Behalf Of WA5CAB at cs.com
> Sent: Wednesday, April 25, 2012 2:26 PM
> To: mrca at mailman.qth.net
> Subject: Re: [MRCA] [armyradios] RE: AB-1241 - DC current rating?
>
> Plus the current rating of the capacitor must be adequate for the
> transmitter output power into 50 ohms.  I.e., you can't use a standard
> disk ceramic.
>
> In a message dated 04/25/2012 13:05:49 PM Central Daylight Time,
> jfor at quikus.com writes:
>> There are two typed of DC blocks...  One has a capacitor in the center
>> conductor only; the other in both the center and shield. You want the
>> former.
>>
>> Look at the spec sheet for the block. You want the capacitor's
>> impedance to be <<50 Ohms at your lowest operating frequency. This
>> easy enough at microwave frequencies, but more of a challenge at HF.
>>
>> -John
>>
>> ================
>>
>>
>>
>> >Yes that is a good suggestion. Would a series cap do the trick (isn't
>> that
>> >whats in a dc block?) And if so, what effect would that have on
>> >antenna impedance and tuning it out?
>> >Thanks
>> >
>> >
>> >
>> >73 Eugene W2HX
>> >
>> >
>> >-----Original Message-----
>> >From: D. Platt [mailto:jeepp at comcast.net]
>> >Sent: Wednesday, April 25, 2012 12:27 PM
>> >To: W2HX
>> >Cc: 'armyradios at yahoogroups.com'; mrca at mailman.qth.net
>> >Subject: Re: [MRCA] AB-1241 - DC current rating?
>> >
>> >I don't think you want to present DC to a virtual short, in any case,
>> >whether or not the transformer or the radio could accept/stand it..
>> >Why not simply add a coaxial DC block to the transmission line and
>> >eliminate the issue, altogether.  Just a suggestion...
>> >
>> >de Jeep - K3HVG
>> >
>> >On 4/25/2012 12:01 PM, W2HX wrote:
>> >>Hi there. I am referring to the "hubcap" stand often used with the
>> >>AS-2259/GR NVIS antenna.  This base has a dc resistance of zero
>> >>because of a transformer within it.  I am looking to use this with a
>> >>radio that has a DC voltage on the antenna BNC, but current limited to
>> about 30ma.
>> >>Anyone have an opinion whether a constant 30ma DC current on this
>> >>transformer would be tolerated by the transformer in the base?
>> >>
>> >>http://www.prc68.com/I/Images/AB1241_PRC104.jpg
>> >>(image courtesy Brooke N6GCE)
>> >>
>> >>
>> >>73 Eugene W2HX
>>
>
> Robert & Susan Downs - Houston
> wa5cab dot com (Web Store)
> MVPA 9480
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