Bird 43 element modification question

[Techman]

I have two 10 watt, 25-60 MHz elements. It seems a bit pointless having two elements exactly the same, so I was wondering about modifying one to indicate a higher wattage. I did a bit of on-line research the other day and there seems to be plenty of information with regards to dismantling and repairing them, but nothing that I could see as regards actually modifying them. I've seen the plethora of methods regarding removal of the top disc and it seems that the usual fault is a bad contact of the wiper of the small preset pot. I note that some of them don't have a pot, but just have a fixed resistor.

I'm wondering what the value of the preset is in the elements that I have here and what range this pot has - that is if it has a pot and not a fixed resistor. I'm only contemplating a power increase modification as I realise that a frequency range modification would be a lot more involved. I've looked on-line at elements for sale and they're mostly over priced tatty examples with dig marks all around the disc and plastered with glue residue, so probably not worth the trouble or risk. It looks to me that a simple increase in the series resistance such as adjusting the pot (which is wired as a simple rheostat) or replacing the pot with a higher resistance one, or adding a resistor will increase the wattage range.

I have to admit that I've not definitely decided to try to modify one of these elements as they're both in very good condition and it does seem a bit of a shame to mess about with one of them, but then there's not a lot of point in having two the same and one of them reads slightly higher than the other, although probably within manufacturers tolerance.

So I was wondering if anyone had ever tried to modify one of these elements and also if anyone knows what the value of the series resistance is and if it's a pot or is fixed in this particular element? I had noted that the pots are often either something like 25K or 14K (if fitted) - any thoughts? Below are some pictures for anyone not familiar with the classic Bird wattmeter, said by some to be the Avometer of wattmeters:-

[Techman]

To clarify things slightly, here is a circuit diagram of how the element relates to the meter - borrowed from the net:-

[Skywave]

Perhaps the length of the short coupling line and its distance from the through-line need to be taken into account.

Three factors to consider: freq., range, power level and conformance to the law of the meter scale.

Perhaps it might be preferable to sell one, add a bit of £ to received sale price and then buy one that you need. I know that they do fetch a tidy sum.

Al.

[Biggles]

I used one of these for years and they were the industry standard. When the meter went for cal, each element came back with a separate cal certificate. I think the main reason for their popularity was the fact that they were tough. Mine survived several accidents during it's life on the road with me, including falls from the top of 19" racks onto a hard floor. I did consider buying one for home use but decided against it and went for one of the very good wide range modern jobs available for about £100. Most Bird 43's will have endured a long hard life by now, and at least one of ours in the workshop had a most annoying intermittent fault on the meter movement. Another used to have the needle stick at various positions on the meter. Mine had lots of paint chipped off by the end of it's service life but battled on. You can get a nice leather case for them too.
Alan.

[G0HZU_JMR]

A 25-60MHz 10W element might be of interest to a CB enthusiast/repairer so I would recommend you try and sell the spare element. This assumes there is a low risk you might blow up the other one at some point in the future.

I've never been impressed with the Bird 43. I've played with a few and all were fiddly and unreliable and the reality kicks in if you get to play with several 'identical' slugs with the same meter body. I saw much more difference between the slugs (at FSD) than that claimed for accuracy for the meter.

I also spent some time testing a couple of them over temperature and the results were woeful. See if you can get Bird to give you a temperature spec for this meter

[Radio Wrangler]

The Bird power meters were reasonably rugged power indicators for use in the field checking things like PMR installations and were good enough for purpose.

They weren't lab equipment. If you wanted accuracy then something from HP, Marconi, R&S came into play.

George Dobbs was looking for something for QRP HF use because a suitable Weltz power meter wasn't going to be exported to the UK, and at the same time the Dunfermline club was setting up a station and needed something directional. So I measured a couple of Bird 43s with the HF slugs and I wasn't impressed. This led me into doing a 4-port transformer hybrid with a couple of diode detectors and a couple of meters. It was published in Sprat 61 and seemed to become a bit of a standard. It was fun to see translated versions popping up from all over the world.

Having two meters, showing forwards and backwards power at the same time is a lot more convenient than a switched single one. Build one with a single meter and you condemn yourself to flipping the switch for the rest of eternity.

Some people used two birds with a slug in each, pointing in opposite directions. But resist the temptation to put a lower power slug in the backwards direction one, There is still full power going through it in the forwards direction.

I suppose that slug might be of interest to someone doing low power on 10m or 6m, but it's not one of the ones in high demand.

David

[Techman]

From what I can make out I think the coupling line is just a single length of wire in the very high frequency elements with more of a multi-turn coil on the lower frequency types. I wouldn’t attempt to try to modify the frequency range of one, although the 2-30 MHz range type would probably be more generally useful than the 25-60 MHz range type. Probably the turns of the pickup coil, perhaps the speed of the diode and certainly the value of the capacitor would be significant in the frequency range of the element, but as far as I can see the ultimate series resistance in the DC output governs the actual indicated wattage output on the meter movement. Having said that, I could be missing some other vital point.

I think that what you’re paying for with these meters is the ruggedness in construction and the advantage of having one meter that can, with the addition of these elements /adapters / multipliers / slugs (chose your own word) cover a massive range of frequency and power. The meter movement is actually resiliently mounted in the case housing with three rubber standoffs where the three holes would normally accommodate nuts and bolts, so as said, they could be dropped from a height and stand a chance of surviving. I think I read somewhere that the inside of the transmission line is actually plated with real silver, whether that’s true or not I don’t know. It’s certainly a precision machined component. The brass cased elements are certainly a good fit, although again, I’ve read somewhere that the latest ones available new are actually brass plated plastic. The clip that holds the element in place is a bit nasty and awkward to release without an ‘implement’ to lever it, although it certainly holds the element in place securely, probably what Jeremy means about being fiddly. Interesting point with regards to lack of temperature compensation. The two 10 watt slugs are a good couple of divisions or so out from each other at full scale. The meter has three slug elements with it and two of them, the 5 watt, and one of the 10 watt ones, look to be of the same older age with heavier printing on the discs, whereas the other 10 watt slug looks like it could be newer with slightly lighter printing. The 5 watt and what looks like the newer 10 watt slug agree with each other, it’s the older looking 10 watt slug that gives a higher indication and is the one that I’d considered modifying. I knew the two previous owners of this meter and I suspect that the original owner perhaps found that the original 10 watt slug had become inaccurate and replaced it, but unfortunately we can’t ask him as he’s no longer with us. The meter was used with low power equipment by the first owner and the second owner used it for CB radio testing repair work

I seem to think that the meter in general reads a little low, but it could be that the meter it’s being compared to is actually reading high. I’ll have to open up the case and connect a low DC supply with large adjustable series resistance and check that the movement is actually reading full scale at 30 microamps. The other point is that I’m not completely convinced that the connecting leads that are with the meter are actually 50 ohms, so I’m going to remake them with known coax. It has ‘N’ connectors like most of them, which means adaptor leads for ‘SO’ so that ‘PL’ types can be used, different sockets are available for the meter housing in America. I think this meter is quite an old one as it has the metal clips to hold the spare elements in the side storage holes and I also note the three screws on each side to hold the case back rather than two each side on later ones, also they’re not cross head types as are later ones. The serial number is in the higher 60 thousands and in a video of the history of the company, it mentions that over 250 thousand have been made and I suspect that this video is probably at least ten years old – link to video at the bottom of this post. I’ve seen meters with serial numbers in the low hundreds, so guess that they might have started at ‘number one’ An interesting comment below this video that hasn’t been challenged, states that the first model 43 wattmeter came out in 1943 (thereby the model number) rather than 1953 as stated in the video and elsewhere. It seems that Ray Bird started the company in 1942. I think the shortcomings with these meters are a bit like the AVO meters, which are far from perfect regarding accuracy and are equalled or bettered by others, but have that ruggedness an reliability that have always made them the industry standard.

As you say, David, the idea of using a second meter reading the reflected power and being careful not to be tempted to use a much lower power slug to be able to see a clearer indication on the scale. I don’t know if it would actually cause any potential damage to the element as even with extremely high powers, the actual currents flowing would still be miniscule compared with the rating of its internal components with my thinking that the actual output to the meter is just governed by the value of the series resistance used, regardless of the power being detected at the pickup sensor coil / wire. However, I may be missing a point somewhere and you could well be right in what you say. I think the more likely disaster would be to have a very low power slug in the reverse monitoring meter and then have a sudden antenna fault which would whack the meter needle to the end of the scale causing possible damage to the movement.

Thanks to all for the comments so far. As it’s turned out, I should probably have titled the thread for general discussion about the Bird model 43 wattmeter as a whole unit, rather than just a question about ‘slug’ modification. I think there’s been very little discussion of this meter on the forum in the past, which is strange considering how universal and long lived they are, so this is probably a bit of a first.

Link to video below:-
https://www.youtube.com/watch?v=KwivYP9FbmY#t=7.831833

[G0HZU_JMR]

I think one of the major compromises in the design of this meter is that it supports slugs down to 100mW.

If you look at the dial scaling you can see that the diode detector isn't getting into the linear region until well past half way. So this implies that the detector diode is always run at a low drive level regardless of the slug rating. So high power slugs will have lighter coupling in order to try and comply with the dial scale.

So having a lightly driven diode detector means you have a diode detector whose efficiency will be significantly temperature dependent over at least half of the dial scale. If you contrast this to a typical CB or ham power meter the scaling on the ham/CB meter usually shows the detector is running nicely in the linear region. Only on the lowest power setting of the meter will the scale show much deviation from linear operation. So a typical ham or CB meter can be expected to be far more consistent over temperature across the meter scale.

I think this is partly why Bird 43 meters are meant to be used up near FSD where the diode detector will be more linear and better over temperature.

Whilst you can rotate the slug around to measure reflected power, this will typically give a reading at a low point on the dial scale and this will mean the accuracy of the meter will be compromised over temperature and also the diode itself will have some efficiency variation from device to device when driven at low dial readings. Also the directivity of the meter is only spec'd at >26dB which means that forward/reflected power measurements can have a fair bit of uncertainty.

To overcome some of the above, the manual for the Bird 43 recommends using a lower power slug to measure power in the reverse direction if you want to minimise measurement uncertainty.

[G0HZU_JMR]

Because the dial scale is calibrated with the diode detector spanning across the Square Law>>Transition Region>>Linear Region any attempt to alter the meter drive to allow higher RF drive level will presumably mean that the lower half of the dial scale will become even less meaningful.

Only the upper part of the scale will be in the linear region so a 'scaled' slug would give the best results only for 'linear' dial readings above maybe 60% of FSD. However, I think quite a few people do these slug scaling mods to try and save on the cost of slugs so plenty of people must be happy with the results they get.

[Techman]

Thanks for your very interesting input, Jeremy. You make a good point with regards to the linear and non linear part of the scale which I had been wondering about. I haven't butchered that slug yet and maybe won't do. It's all food for thought.

[Techman]

Well, there was no need to try to modify a slug after all. I picked up a 100 watt 25-60MHz, 100A at the recent Newark Ham fest, for a tenner - a third of the price the complete meter with case and accessories cost in the first place! Obviously I realise these meters are worth a lot more than that, so all's good. The replacement slug has obviously been worked on, as the cover disc is missing, probably stuck back on using the residue of the glue, but fell off and got lost. Anyway, it was said to be in full working order, which indeed it was.

The pictures below show the slug having glue residue which I'll try to clean off with some solvent. I'll perhaps make a disc to fit as there's zero chance of obtaining a replacement. Maybe there would be a way to reproduce a scan of a correct disc that could somehow be transferred?

I also invested in a couple of 'N' to 'SO' adapters while I was there, which will be better than those made up leads which I'm not sure are actually 50 ohm anyway. I've pictured below some of the markings on the various leads and that white one doesn't say 50 ohm as far as I can make out - any ideas?

A Bird 43 wattmeter isn't really something you need, it's just one of those things that's 'nice to have' and will probably spend most of its time living on a shelf in the cabinet next to the various Avo's, but will definitely get some use on the odd occasion.

[Techman]

Originally Posted by G0HZU_JMR

I think quite a few people do these slug scaling mods to try and save on the cost of slugs so plenty of people must be happy with the results they get.

Do you think that there are actually a lot of people that modify these slugs?

I did a bit of searching and couldn't find any reference to actual slug modification, only people dismantling non-working slugs to repair them, hence the reason that I posed the question on actual 'modification' in the first place. Perhaps I didn't look hard enough?

If I happen upon another slug of the 100A type in better condition at the right price then I've got this one that's already been opened to play with. Having said that, the HF range from 2-30 MHz type would ultimately be more useful, but I'm happy with what I've got - certainly for the price!

I've found out that there are actually 'collectors' of these 'slugs' - amazing!

The slug I have just obtained has cleaned up nicely, so I may not bother with a disc at this stage.

I like the 'genuine top grain cowhide' slogan on the bottom of the case - none of this just calling it 'leather'.

[Techman]

I opened up the meter itself to test the movement at FSD. By the look of the screws and the paint under their heads, it didn't look like it had ever been opened up before. The meter read full scale at 30uA, maybe the width of a needle low depending how you lined up your eyes with the scale, so definitely well within tolerance I would say.

I decided to open up one of the 10 watt slugs as they're not worth that much being just the top end of HF, the full HF range slugs being more desirable and worth a lot more, so not too much lost. I decided to open the one which seemed to me to be reading a little on the low side. As can be seen, there's no calibration pre-set pot, just whatever is inside that piece of sleeving. I'll be investigating this further!

[Techman]

Well that's proved that! As the pictures below show, there's a high stability resistor of 17.5 K, which due to aging is no longer very accurate with its value rising to 17.74 K.

That sort of difference probably wouldn't make a lot of difference in many pieces of kit, but in something like this it's critical. I think that this proves that with a standard component (although perhaps not a very standard value) like this, then it's a 'given' that it's going to alter in value with age, and more than likely to rise rather than fall.

I'm glad I opened this 'slug' as it was certainly no good as it was, other than an ornament! I bet that most of these 'slugs' you see around are inaccurate and little more than 'indicators' if they haven't been 'looked at'. I think the one I bought recently has actually been re-calibrated by someone, as it seems to give the consistent readings that I would expect to see. I think the 5 watt slug is also a little low in reading, as it matched the 10 watt one I've just opened, the other 10 watt slug near enough matches the 100 watt one.

I did some experimentation with a pre-set pot, filed slightly to fit and also a resistor of another value, and have basically proved what Jeremy stated in his post #9 of this thread.

I opened the top of the 100 watt slug just to have a look inside and noted that that it had the expected pre-set, but there would seem to be some circuitry difference between the two slugs. The 10 watt slug with fixed resistor is making three electrical connections, while the 100 watt slug with pre-set is making two electrical connections in the top of the housing.

There must be a way of re-scaling these slugs, but I have to confess that I don't have enough knowledge on these things to really know what else to alter, or even whether to bother to try However, I do now have this opened up 10 watt one to mess about with now and I could certainly use the disc off it to tidy up the 100 watt one - just adding another '0' to the labeling