Bird Thru Line and slugs - Page 1
Forum section: electronics / rf-microwave

author: CJay
author_rank: Super Contributor
timestamp: September 13, 2016, 02:08:07 pm
content: Given the multitude of handy little log detector ICs and even simple things like a dummy load with built in diode probe that can be made into rather accurate RF power measuring devices, what are the advantages to a swinging needle meter which has a full scale error of 5%?They seem to have a mythical status among hams and CB users, what am I missing or am I right to be sceptical of the high esteem the old tech* Bird meters are held in?Other than the ruggedness of them can they 'compete' with the newer tech?*Old Tech not being an insult, rather a means to differentiate the newer digital Bird meters from the Thru-Lines

author: dmills
author_rank: Super Contributor
timestamp: September 13, 2016, 04:14:11 pm
content: The things were (and are) bomb proof.Sure there are better things, log amps among them for some jobs. But the Bird can live in the toolbox and you can pull it out after a year or so with no concerns about flat batteries (and little concern about overload). It will (with the correct slugs) cover from LF up into the low microwave from 100mW or so to at least 5kW.It is like the Avo 8 (Sometimes described in the UK as Gods own multimeter), there are more modern things with shiny features and much higher input impedance, but these old tools just keep on keeping on, I have a model 8 that is as old as I am, and it is still more immune to RF then most modern meters (Plus it has a 3kV range!).5% is more then good enough most of the time, if it is not, you can very often take that as a sign that you are doing it wrong.Regards, Dan.

author: w2aew
author_rank: Super Contributor
timestamp: September 17, 2016, 10:02:25 pm
content: ...and perhaps more importantly, the bird elements/meters are directional, so you can measure forward and reflected power directly.

author: G0HZU
author_rank: Super Contributor
timestamp: September 17, 2016, 11:26:35 pm
content: I have to say that I don't share the same enthusiasm about the Bird 43 meter.It's best features are (as already mentioned by dmills) that it is potentially very versatile and can be used across a huge frequency range and power range if you buy enough slugs. The through loss is very low and the VSWR of the meter is very low as well right up into the GHz region.It's most suited for casual ham use or for doing very basic field testing where its limitations aren't that important.One of the main issues with it is that it operates the detector at quite a small drive level and this means that the diode detector is prone to errors wrt temperature when used at small/mid deflections on the dial. The detector isn't temperature compensated. Also, my experience indicates that the slug connection is far from bombproof and often the slug needs to be wiggled or cleaned before use. In the past I've played with two Bird 43 meters side by side and a trio of identical slugs and tried swapping between them. For the same test tone I saw errors of -5% ranging down to -20% when driving up near FSD on the dial. It all depended on which meter was paired with which slug. But there were large/obvious errors between the slugs even on the same meter. Plenty of other people have had similar experiences.Also, the useful range of the slug is only a few dB assuming you want reasonable accuracy over temperature. It's easy to see changes in power reading of 20% if used in a hot environment compared to very cold when used at low to medium deflections on the scale. So be careful when using it for outdoor field trials on very hot or very cold days.Finally, it relies on the user presenting it with a clean sinusoidal test signal with very low harmonic levels. This type of meter can give very large errors if harmonics are present. i.e. errors much larger than you might expect when you compare the Bird 43 to a power meter that can measure average power correctly. For typical ham/CB use you can argue that most radios will not generate harmonics at levels high enough to cause serious errors but (in theory) even harmonics at -30dBc to -40dBc can begin to influence the measurement uncertainty of this meter. So be careful if you try and use it to test a wideband RF amplifier that has little or no harmonic filtering.

author: dmills
author_rank: Super Contributor
timestamp: September 18, 2016, 07:30:37 pm
content: For RF measurements +-10% or so is however often good enough, the 43 is the carpenters tape to metrological tool that is the HP and a stack of calibrated attenuators.Both very much have their place but when I am wanting a quick check that the amp is working and the aerial has not fallen down I know what I usually reach for.The good RF power heads tend to be both expensive and delicate, accidentally putting 10W or so up my HP power head would make for swearing (less then 10mW to saturate that thing), the bird? No worries mate, carry on.Regards, Dan.

author: G0HZU
author_rank: Super Contributor
timestamp: September 18, 2016, 09:24:24 pm
content: Yes, it's fine for casual use etc.The reason I chimed in is that discussions about the Bird 43 tend to get wrapped up in fairly meaningless hyberbole by the people who think this meter is something special.So you get people saying that it's been the "industry standard for decades" or "it's built like a tank" or "there's watts and there's Bird watts"All of which hide from the fact that this is a very crude device under the cover and the concept is flawed due to its own versatility. I'd argue that the majority of CB/ham users who operate at power levels below 200W would be much better off with a power meter made by one of the regular ham power meter' manufacturers. Cheaper, more stable over temperature and it will probably be better at measuring FWD/REV power ratios accurately.The Bird 43 spec says it has a directivity spec of just 26dB. I'd imagine that some slug variants will be much better than this but 26dB directivity is not good if you want to measure VSWR with reasonable accuracy.Also, even Bird state that the uncertainty/accuracy at low needle deflections (presumably due to spreads in diode characteristics and temperature) means that it isn't really suitable for serious use as a VSWR meter unless you use multiple slugs to do the FWD/REV measurements. That fiddly ritual of slug swapping doesn't sound like fun to me.I've been involved in RF/electronics ever since my student days and I've got some nice RF test gear here but a Bird 43 wattmeter isn't likely to ever be on my shopping list. For casual stuff, I have a cheapo Welz power meter for any time I might want to mess with antennas on the ham or CB bands and I wouldn't swap it for a Bird 43.

author: Flump
author_rank: Frequent Contributor
timestamp: September 19, 2016, 08:53:27 pm
content: I dont own a Bird 43 but I have used a friends on several occasionsand as rugged as they are they have off days where they dont registerat all or they give a highly attenuated reading.For low level work I use a Fluke RF probe but it only measures up to around 18W and I think only covers HF.I would love to build a 3in1 or 3 separate meters 1-10w , 10-100w , 100-1000ideally they would cover HF up to 2M but even a set for HF and a set for VHF would be nice.but It's a bit above me and I looked in to it but never felt confident enough to do it.I have many SWR / RF Power meters and to be honest I find myself usingthe Avair 20 & Avair 40 the most.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 20, 2016, 02:17:39 pm
content: Bird has diversified into other RF products for a reason.In the US, the Bird Wattmeter is a military specified product, and those tend to stay around for decades. Technicians learned on a Bird, and they go out and buy Bird.. Think of it as tradition and market saturation.Besides that, like the others, I'm not hauling my fragile, expensive HP up a tower. My temperature compensated handheld Daiwa is no longer made.I'm looking at the new digital stuff and it all seems to be in bulky, clunky packages, it looks like no one makes a compact analog VSWR meter any more.Meter movements have a definite use when measuring SSB RF, too...So market forces dominate technical sense in this case, and the bulky, simple, things have won. The hams that have big desktop rigs want a big desktop meter to match the décor...Steve

author: AF6LJ
author_rank: Unknown
timestamp: September 20, 2016, 03:29:12 pm
content: In the real world 5% is good enough.

author: CJay
author_rank: Super Contributor
timestamp: September 26, 2016, 08:29:50 am
content: Thanks for all the replies, you've all pretty much confirmed what I suspected, that the ruggedness and inertia of the human condition make these the 'go to' for many users.As some have said, 5% is good enough most of the time and yes, more accurate is nice but not essential.However, for the price of a Bird with the necessary slugs, I'll stick with my dummy loads/diode detectors (thick film RF load variants and bunched resistors) and the log amp homebrew meters and be reasonably happy I'm in the same ball park.I like being able to measure sub milliwatt with the log amps and up to 250W (probably in excess of) with the thick film load so unless a Bird falls into my hands for very cheap/free I'll not worry about them.