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Build a simple RF sampler for the Bird
Model 43 Thruline Wattmeter
By Kevin K. Custer WJ8G
This construction article will help you take a dummy slug (more correctly called the dust / safety plug) and with some simple drill press machining, build a very useful RF sampler / Iso-tee for feeding a service monitor, spectrum analyzer, etc., and for a lot less than what Bird wants; (over $100 at the time of this writing!) If you don't have a dust plug, one can be obtained from Bird directly or a stocking dealer like RF Parts; Dust / Safety Plug, Part# 3610-031, about US$9.00 each in early 2012.
Concept: The Bird Model # 43 Thruline Wattmeter is a very common test tool among Repeater Builders worldwide. This article will show you how I took a simple dummy slug and turned it into a very useful tool for testing for desense, monitoring waveforms, and feeding other equipment. I've tried to make this article as detailed but concise as possible to benefit readers. This modification process will not disturb the original intent of the dummy slug; keeping out the dust and shunting the meter movement in travel. Although this article was written around the modification of the Bird 43 wattmeter components specifically, the information contained within this article can be used to modify any similarly manufactured equipment for like use. If interested, I provide the service of making this item for those not having the tools or time to do it, Click Here for pricing, pictures, and more information.
Construction: Review the pictures
and machine the aluminum dummy slug by boring a hole, of the appropriate
size, right through the face of the slug and out the back (line bore).
The hole size should be what is called out for the thread tap that matches
your BNC Female connector. Countersink if desired. You'll also
note the additional hand machining of the rear surface to more easily allow
the routing of the center conductor, however, this step is an option.
Tap the face side for the the correct thread size to allow attachment of the BNC female connector, but do not install it yet.
Solder a 47 (50) or 51 ohm carbon resistor to pin, and then install the connector into the slug. Make sure the lead is of the correct length; as it should be just long enough to allow the body of the resistor to reach the back face. I used some insulation from a piece of wire to insure the center conductor doesn't short out. Orient the resistor to lay as shown in the pictures. A small hole can be drilled into the body of the slug to accommodate the ground connection. If you choose to drill a hole, the use of a center punch will swedge the hole shut and squeeze the resistor lead for a solid connection; (Pittsburgh fit). I didn't have a real small drill bit, so I used the smallest one I could find and doubled the resistor lead over to more completely fill the hole. I drilled the small hole inside of the line bore very close to the rear surface (back face); which allowed me to use a small pneumatic center punch with excellent results, (see the bottom picture).
Option: Fill the hole on the resistor side with hot glue to hold
all of it in place.
You can turn the pick up slug to vary the coupling if desired.
NOTES: The following notes have been offered
by Allan Crites - WA9ZZU
When using an iso-tee in this instant application it is highly desireable to use a 50 ohm termination at the iso-tee to properly terminate the signal source. Using a modified and terminated Bird Wattmeter slug for signal sampling is a highly desireable means for observing off the air signals, for observing desense, and transmitter output. However, there is the possibility of the transmitter output entering the signal generator and creating intermod if used for the purposes of making a desense measurement and should be seriously considered. I recommend the use of a 10 dB attenuator on the BNC output to the signal generator to lessen the possibility of intermod. While rotating the slug reduces any transmitter signal it also reduces the coupling that would be required for the signal generator to overcome. Another way to reduce the coupling would be to not insert the slug as far into the WM, thus having the same effect/benefit as the iso-tee isolation with the added characteristic of a 50 ohm termination on the BNC connector. This tool has the added benefit of directional magnetic coupling whereas the iso-tee is capacitive and does not. AC
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Written and HTML'ized by Kevin K. Custer WJ8G
Artistic layout and hand-coded HTML © Copyright August 26, 2005 and date of last update by Kevin K. Custer WJ8G
This page last updated January 2012 - Dust Plug pricing from RF Parts.
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.