[Milsurplus] [ARC5] Smart People: Antenna Matching Mystery

Mon Aug 7 16:09:14 EDT 2017

Just want an idea of what type of RF current is across the capacitor, don’t need to be complex. But I am going to assume that if the radio is developing  around 0.9 amperes of RF current and P=(I * I) * R  that would be around 40 watts. And at about ¾ of an Amp that would be just under thirty watts. This is all assuming 50 ohms with an ideal load. So is it that you are developing significant heat with currents as low as or under one amp?
Unless you’re dealing with some weird power factor thing from the L52 and new capacitor combination the voltages are below one hundred or so volts so back to the original question, why dose the capacitor get hot? I know engineering can get complex but this is about the simplest output tank out there.

Ray F/KA3EKH

From: milsurplus-bounces at mailman.qth.net [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of MICHAEL ST ANGELO
Sent: Monday, August 07, 2017 3:36 PM
To: Michael Hanz <aaf-radio-1 at aafradio.org>
Cc: milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] [ARC5] Smart People: Antenna Matching Mystery

I recall a document detailing the load impedance of antenna on various planes. Does anyone have a copy of that document for the Command antenna for those planes?

Once we have a range of impedances the set is supposed to match we can design an L network to go from that impedance R + jX to our antenna load.

Mike N2MS

On August 7, 2017 at 2:43 PM Michael Hanz <aaf-radio-1 at aafradio.org<mailto:aaf-radio-1 at aafradio.org>> wrote:

Peter is correct.  But what he didn't say was how easy the explanation will be, or how long it would take to teach you, or even if you would feel any satisfaction at the end.  RF design, and transmission line and antenna design in particular, is sort of a specialty area in EE because so many other factors start showing up at different rates with frequency.  The differential equations start going nuts.  They mess up the nice, well behaved lumped component view and math that you get in Physics 101.  You either love the complexity of the specialty, or you don't.

RF engineers have a different frame of mind - a sort of other-worldly look in their eyes, peering into the depths of space as if they can see something several light years away.  Okay, I made that last sentence up in fun, but from a specialty choice standpoint, it does take a slightly different talent than, say, circuit design or power distribution.  You can get a whiff of the problem by reading all the antenna theory battles that go on endlessly on the web.

73,
Mike  KC4TOS
On 8/7/2017 1:46 PM, Ray Fantini wrote:

A statement like that almost makes me want to say, Ok so explain it to people like me who don’t know! Being of relatively small mind and limited capabilities myself am left to looking at what’s been out there in the printed world.  Looking at the basic output tank it’s a straight forward parallel resonate circuit with T54 also serving to match the high impedance of the plate tank to the low impedance of the antenna. The kind of thing a transformer works best at. L 52 adds inductance to the output to compensate for the short electrical length of the antenna and that’s about all there is to it. No suppression of unwanted harmonics or anything else just a great example of fast and expedient engineering.

Like I said I am not that smart so if I got it wrong feel free to correct, most of my knowledge of the inner workings of the ARC-5 family beyond my limited experience was gleamed from the CQ Surplus conversion Manual or better yet the $1.50 Command Set compendium published back in 57. Some of the later articles in the book outline how to modify the set for TVI proof operation but almost all of the modifications for Ham use first involved removal of L52 the roller inductor and directly connecting to the secondary of T54

I know conversion is a dirty word around these parts but much can be learned from that book. This is a web link to it:

http://hilltoparmyradios.com/command_sets.pdf

Would be real advantageous for one of the smart people to run a software model and see just what type of voltage is developed across that capacitor and better yet what RF current. Assume an output load 50 Ohms and L52 set to 0, minimum inductance. Think it may be surprising!

Ray F/KA3EKH

From: milsurplus-bounces at mailman.qth.net<mailto:milsurplus-bounces at mailman.qth.net> [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Peter Gottlieb
Sent: Monday, August 07, 2017 12:54 PM
To: arc5 at ix.netcom.com<mailto:arc5 at ix.netcom.com>
Cc: ac2eu at yahoo.com<mailto:ac2eu at yahoo.com>; arc5 at mailman.qth.net<mailto:arc5 at mailman.qth.net>; milsurplus at mailman.qth.net<mailto:milsurplus at mailman.qth.net>
Subject: Re: [Milsurplus] [ARC5] Smart People: Antenna Matching Mystery

It's not voodoo, just electrical engineering, and pretty well understood.

Peter

On Aug 7, 2017, at 11:50 AM, arc5 at ix.netcom.com<mailto:arc5 at ix.netcom.com> <arc5 at ix.netcom.com<mailto:arc5 at ix.netcom.com>> wrote:

Sent from my Ain't Smart Phone.

------ Original message------

From: J Mcvey

Antenna Matching Mystery

...The 50 ohm coax at the terminal causes a host of reactance and reflection issues by itself.

Bet if you added coax length, the situation would change to something else, maybe better, maybe worse!

RF is spooky stuff!!

----

You are so right.  I switched the coax jumper to a long one, coiled into a big spool "shield RF choke."  It completely changed the tuning.  RF is Electronic VooDoo.

73 Dave S.

On Monday, August 7, 2017 10:13 AM, Ian Wilson <ianmwilson73 at gmail.com< a="">> wrote:</ianmwilson73 at gmail.com<><mailto:%3cianmwilson73 at gmail.com>

I have noticed that the roller coil has distributed capacitance, so it

does not always behave as a simple inductance even at lowly RF.

To a reasonable approximation, it could probably be modelled as

"a few" L-C sections. 4 would be a good starting point.

73, ian K3IMW

On Mon, Aug 7, 2017 at 6:57 AM, David Stinson <arc5 at ix.netcom.com<mailto:arc5 at ix.netcom.com>> wrote:

OK y’all- here’s a mystery (at least to me),

Series circuit, designed to match a normally 5-12 Ohm

Aircraft radio output to resistive 50 Ohms.

Follow me here:

Series circuit:

PA Tank coupling link – Roller coil –

120pFd ceramic cap mounted at antenna post –

50 Ohm Load = 25 W out.

PA Tank coupling coil –  120pFd ceramic cap mounted

at the input of the Roller Coil – Roller Coil –

50 Ohm Load = 15W out.

The ceramic cap is near chassis ground both ways,

so it’s not circulation currents.

The PA tuning cap setting does not change,

so it’s not de-tuning the PA tank.

Why the difference between these two series circuits?

73 Dave AB5S

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