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+--- Thread: Bird slug reads high (/thread-541.html)
Bird slug reads high - VK3ALB - 22-03-2018
Hi All,
What do you think would make a bird slug read high?
Slug is a 50D - 200MHz to 500MHz. The test setup is proven, two meters in series connected to a Termaline load with meter.
Meter tested ok with known good slugs.
Slug behaves badly in known good meter.
Slug reads FSD (hard over!) in the FWD direction.
Slug can be adjusted to make sensible readings when rotated to reverse. I think this is pointless.
Terminating resistor and diode replaced, trimpot works ok, no dirty spots, smooth adjustment.
I don't know the answer but what do you think would make a bird slug read high?
RE: Bird slug reads high - VK5PJ - 22-03-2018
Hi Lou,
can you please confirm the EXACT numbers on the slug, and its suffix, there an a number of slugs getting around from Ex-TV sites that are designed for 100 uA meters not the usual 30uA meters we see in the 43 series.
an example of one I have here is a: 1000E1 , which is 1000W 400-1000MHz but the telltale is the number after the letter which designates it as to be used in one of the big rack mounted systems with 100 uA meters.
just a thought..
Peter, vk5pj
RE: Bird slug reads high - VK3ALB - 22-03-2018
Hi Peter,
Yes thought about that but it's difficult to tell by looking at the insides. The label on the top of the slug says 50D and the texta marking inside the top also says 50D. That we get a reading in both directions when connected to a good load is strange.
RE: Bird slug reads high - VK3PY - 23-03-2018
Lou,
There isn't much inside a Bird slug, as you know. I'm curious why the diode and terminating resistors were replaced. Did you repair the slug yourself?
There is obviously too much coupled voltage produced by the slug. About all that I can think of is that (a) the coupling of the sampling line to the main line is way too close, as if it were maybe a 5D slug, or (b) the terminating resistor's value is much too low. How was its value determined? Presumably it was open-circuit, which is why it needed replacement.
For what it's worth, I disassembled my 50D slug and measured the fixed resistor's value. It is 18.4k. I hope this helps. I've attached a photo for your perusal.
Chas
VK3PY
RE: Bird slug reads high - VK3ALB - 24-03-2018
Hi Chas,
The slug belongs to Matt VK3PP and came with the meter he bought on an auction site.
You image is a great help and looks nothing like the insides of this slug. The loop is much different and sits a lot closer to the line.
Parts were changed out of exasperation rather than science or logic. The terminating resistor was marked and did read 68ohms. The previous owner (unknown) left a screw out of it so we knew it had been tampered with.
Replacing the terminating resistor with a 18K meant we could adjust it to read approx half scale with 20W in however we no longer had directivity. I guess we have a low power slug that has been mis-labeled.
Like you said, only four parts how hard could it be. Matt has gone home now but we'll keep looking at it when we catch up next.
RE: Bird slug reads high - VK3PY - 24-03-2018
Lou,
Re-reading your initial posting, you mention the trimpot works OK. What trimpot??? There is no trimpot in my 50D element. Its circuit diagram is as shown in the accompanying schematic. The terminating resistor "R" measures as 75 ohms. It in inside a small sleeve that forms an adjustable, shunt compensating capacitor. Resistor labelled "82" is 18.4k.
I suppose calibration is performed by carefully adjusting the coupling of the auxiliary arm in the slug.
Chas
VK3PY
RE: Bird slug reads high - VK3ALB - 24-03-2018
I think a picture is worth 1000 words. I'll take a couple next week when Matt comes back.
Now that I'm awake I see you have shown me the top compartment of the slug. I venture inside that tube is a trimpot, possibly 25K.
We used this http://www.repeater-builder.com/projects/bird-element-tour/bird-element-tour.html as a reference.
There is an image of the sense loop of a 100E slug. That looks similar to the loop in our 50D slug.
RE: Bird slug reads high - VK3PY - 24-03-2018
Lou,
You wrote: "I venture inside that tube is a trimpot, possibly 25K."
Nope. No trimpot. Re-visit the schematic diagram. There is a tube with an adjustable sleeve (shown as distributed capacitance in the schematic diagram) in which a FIXED resistor resides.
Something that has become evident in my recent investigations is that Bird slugs differ markedly in their detailed workings, depending on band and power level.
Chas
VK3PY