Jim,
Is that really comparing apples to apples?
In the case of the bandpass filter, you have a (hopefully) 50 Ohm signal
passing through the coil.
Considering that the bandpass filter is inserted in between the TX and
Linear, we are talking 60 to 100w power passing through these.
In the case of the balanced matchbox used for matching antennas anywhere
from 10 Ohms to 2000 Ohms at power levels up to 1.5 kW, with all kinds of
reactance, it seems to me that any toroid might have more problems with
heating than a hefty air (core/cooled) coil.
At low power levels of 100w or less and near 50 Ohms all the time, I'm sure
you are perfectly correct but in a high powered matchbox I fear for the
toroid. In practice, using commercial toroid baluns between an asymmetrical
matchbox and an openwire fed dipole in contests, I have burned up several
baluns which were rated 2.5 kW, and I have never run more than the legal
limit here (750w). That's why I stopped doing that about 10 years ago.
Inside the balun I often found everything melted.
Using big link coupled matchboxes such as the J.V. Kilowatt or the Big
Annecke Symmetrical Koppler, I have never had the least bit of problems and
just for kicks, I even opened the matchboxes after some hours of operation
and felt the coils to see how hot they were getting. They ware warm, not
hot. But both of these have BIG coils.
Like Bob, I prefer air-wound coils in my matchbox.
However one must be aware that there are cheap aircore coil with simple
plastic insulation and there are better coils with a special poly-carbonate
insulation which handles heat a lot better. You must choose the right ones.
The cheap plastic ones tend to deform when they get warm. The cheap
matchboxes I've seen, all use the cheap plastic coils.
73
Rick, DJ0IP
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Monday, February 18, 2013 7:34 PM
To: tentec@contesting.com
Subject: Re: [TenTec] OT: Openwire Balanced Antenna Tuners (QST Test)
On 2/17/2013 5:54 PM, Bob McGraw - K4TAX wrote:
> I'll take an air-wound inductor over a toroidal inductor any day,
> space available. The typical loss in an air-wound inductor is IR loss
> where a toroidal has IR loss plus eddy current loss.
That may be too broad a generalization. While I haven't crunched the
numbers for specific designs, I recently completed VNWA measurements of all
the bandpass filter boxes I could get my hands on for multi-transmitter HF
stations. Some use air-wound inductors, some do not. The filters having the
lowest loss and the best performance are the W3NQN designs, both those sold
by Array Solutions buit by W3NQN, and those implemented by Bob, 5B4AGN, in
his TXBPF kits. Those designs use two different mixes of powdered iron cores
for the lower bands, and non-magnetic toroidal cores (essentially a PVC coil
form) for 15 and 10M. The W3NQN designs also vary the wire size from one
band to another.
My report, which includes photos of the filter sets and lots of measured
data, is at
http://http://audiosystemsgroup.com/BandpassFilterSurvey.pdf
73, Jim K9YC
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