Steve & All: One weekend last winter I decided to play around with an
Elecraft CP-1 dual directional coupler kit which has 25 and 250 watt power
ratings depending on how you build it for 1-30MHz (20 and 30 dB coupling
options.) I decided to try to modify the high power version designed for 1
to 30 MHz that I previously built which has a 30dB coupling ratio. It uses
a pair of FT50-61 cores with 31 turns as the coupling links. I simply
changed the FT50-61 cores at 31turns each to FT50-75 cores with 31 turns
each. The 30 dB coupling factor remained essentially unchanged from 50 kHz
to 500kHz and the return loss into a matched load 50 ohm was > 30 dB. This
coupler kit has switchable 2W 5% 50 ohm load resistors as well as RF coupler
outputs. I use the RF coupler port outputs and the best (surplus ebay)
precision loads I had available to test it out. I have not ran any LF/MF tx
power through the modified CP-1 coupler but I use it regularly to check the
match on my MF/LF EWE antennas, rx loop antenna projects, etc. The pdf
manual for the CP-1 is on the Elecraft web page to review the schematic.
What I learned from this exercise was that the coupling factor remained the
same for a given number of turns on the coupling toroids in the CP-1
coupler. Apparently one can change the core material to shift the frequency
range according to one's needs. One of these days I should run some tests
with 77 and 43 cores and compare the results. Anyway, the test results with
the FT50-75 cores in the CP-1 coupler are in the dropbox link below...
https://www.dropbox.com/s/khtl9m4eoyc0cr9/ELCRCP1.jpg?dl=0
When I get around to building an LF/MF transmitter I will likely use the
modified CP-1 and keep an eye on the core temperature with a contactless IR
thermometer to determine the usable power limit....
73, Mike wa3tts
----- Original Message -----
From: "Steve" <[email protected]>
To: <[email protected]>
Sent: Wednesday, November 02, 2016 5:36 PM
Subject: Re: LF: Re: SWR Bridges
Tnx Pete and Marco...yes, I agree that using an RF ammeter to set
impedance match, once resonant, is a simple way for those that are without
a scopematch. For most LF/MFers however, it is only a matter of time
before they construct the scopematch!
Steve 73
Hi Steve,
wish to add just one comment sharing with you my experience.
For routine tests or measures on antennas since almost a dozen of
years use an home made VNA (I realized the N2PK VNA and still works
fine!) this make available visual and numerical datas which very
important to understand what I'm doing. As everybody, of course, I like
to have an instrument in line while transmitting just to see is
everything is "normal" and under control. For this purpose I made a
small console which includes the low pass filter, an RF ammeter and an
SWR bridge is very easy to design and make yourself such instrument I
made mine covering from 137kHz to 160m and I guess it is reliable at
least as all "in line" meters.
73 de Marco IK1HSS
----Messaggio originale----
Da: [email protected]
Data: 2-nov-2016 19.12
A: <[email protected]>
Ogg: LF: Re: SWR Bridges
Many thanks to all of those taking the time to reply to my inquiry. It
seems
that results vary depending upon what meter is being used but it sounds
as
though only a few are having reliable / accurate results. I normally
use the
scope match and call it the 'most valuable piece of gear in my shack'
but
was curious about suggesting the use of an HF SWR bridge for those that
do
not yet have such a thing, and if it would be a reliable method of
matching
an antenna. In some cases 'yes', but it seems, in most cases, may be
unreliable and only lead to confusion.
Steve 73
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VE7SL BLOG - "Homebrewing and Operating Adventures From 2200m to
Nanowaves": http://ve7sl.blogspot.ca/
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