[Top] [All Lists]

[TowerTalk] Line Isolators (was: common mode chokes)

To: <>
Subject: [TowerTalk] Line Isolators (was: common mode chokes)
From: (Chuck Counselman)
Date: Fri Jun 27 17:12:12 2003
Seeing "line isolators" in the same message as [power] "amp" compels 
me to warn everyone:

(1) Don't put a Radio Works "Line Isolator" in the output line from a 
legal-limit HF power amp.  It's unsafe, as I shall explain below. 

(2) The common-mode choking impedances of these products are not as 
advertised.  I'll explain this assertion, too.

First, regarding power:

After burning out not one, but two, Radio Works model T-4 Line 
Isolators, I was ready to complain and demand my money back, until I 
read the fine print on their web page.  That page and its URL have 
since changed; the current URL is 
<>.  On the current page 
it is stated that the power rating of the model T-4 is "1500 w"; and 
a footnote states "Power rating is based on a normal SSB or CW 
duty-cycle and an SWR of 3:1 or less at the output of the Line 
Isolator. Line Isolaotrs [sic] are not rated for high power AM, RTTY, 
or any high duty-cycle digital more operated at high power."

I don't see it stated on the current page, but on the old one it was 
also stated that this power rating was for a frequency of 3.5 MHz, 
and that one must "derate" at higher frequency.

The current page does include a link 
<> to page 29 of a 
"Catalog" in which it is stated in the table at the top of column 1, 
"Derate power at 28 MHz."  The qualification "SWR of 3:1 or less" is 
omitted -- at least it's nowhere on the referenced page.

Let's assume that the SWR on the coax at the output of your power amp 
is 3:1 on all bands.  Let's assume further that you never transmit 
with more than a "normal SSB or CW duty-cycle."  The power _loss_, or 
_dissipation_, in this product increases _at_least_ in proportion to 
the square root of the radio frequency because the dominant cause of 
loss is resistance in the center conductor of the coaxial cable of 
its winding.  (I wrote "_at_least_" because there are thermal-runaway 
and resultant insulation-decomposition and dielectric-breakdown 
issues as well.)

So, even in your idealized circumstances, for 20 meters the power 
rating is 750 w; and for 10 meters the power rating is 375 w.

Does your power amp put out more than these rated powers?  I thought so.

As mentioned, I burned out two model T-4 Line Isolators.  I was 
transmitting 1500 watts on 20 meters (Oops -- double the rated 
power!) and kept the key down long enough to tune my amp. (Yike! -- 
another no-no!).  The SWR in my coaxial feedline was about 2:1. 
Suddenly, and fortunately, the SWR alarm of my RF Applications Inc. 
digital peak-indicating wattmeter went off.  I promptly QRTd, 
imagining an arc in my remote antenna tuner.

Investigation revealed that the Line Isolator in my attic, 70 ft. 
away, had shorted.  It wasn't hard to find the fault.  My attic was 
full of smoke, and the outside of the Line Isolator's PVC plastic 
enclosure, originally intact and all-white, was now severely 
deformed, partly burnt, and shades of brown and black.  The inside 
was almost totally burned-out and charred.

If I had not been so quick to unkey my amp, I could have lost my 
house.  An attic fire is the worst kind.  I would not have have 
discovered it until after the house was doomed.

The second T-4 Line Isolator was in the same coaxial feedline but in 
my shack.  However, it was out of sight, behind my desk.  In light of 
what I'd found in the attic, I pulled out the second Line Isolator. 
Its PVC plastic case was deformed and partly brown.  I cut it open. 
Inside were the melted and charred remains of some foam; and the 
jacket of the coax was charred.  The inside of the coax also showed 
heat damage.  However, the coax had not shorted.

I have a digital photo of the inside of the second Line Isolator that 
I can send anyone who's interested.  (A few of you have already seen 
it.)  My wife made me throw out the first one 'cuz it was smelling up 
the house.  I salvaged the ferrite rod and threw out the rest.

Now regarding common-mode choking impedance:

The web page (URL given above) states that the "Isolation @ 14 MHz" 
is "80,000 ohms."  The catalog page (ditto) states "Winding Z @ 3.5 
MHz >33K" and "Winding Z @ 14 MHz >50K".

I measured the impedances (between the ends, coax shield to shield) 
of my Line Isolators before I installed them.  Between 3.5 and 14 MHz 
the Line Isolator looks like a very high-Q inductor with an 
inductance of about 38 microhenries.  In other words, the impedance 
has an essentially zero real (resistive) part, and a positive 
imaginary (reactive) part equal to 2*pi*f*L, where L = 38 uH.

At 3.5 MHz, the magnitude of the impedance is equal to about 836 ohms.

That's _less_ than 1 kohms, not ">33K" [ohms], as advertised.

At 14 MHz, the magnitude of the impedance is equal to about 3343 ohms.

That's about 3.3 kohms, not ">50K" [ohms], as advertised.

I asked the proprietor of Radio Works about this by email.  After 
initially replying to suggest that perhaps I'd been sent the wrong 
model, he ceased responding to my email.

I invite anyone owning one of these products to measure its impedance 
and to report your results on this reflector.

Caveat emptor!

73 de Chuck, W1HIS
<Prev in Thread] Current Thread [Next in Thread>