DIY RF Condenser Mics

[khalidgul]

V.5 of the schematic draws less than 3mA from the phantom power supply in total, so even a relatively 'weak' supply should function OK.

Using a DMM you should be able to measure voltages within the following values at these points, relative to gnd (pin1):
• Junction of R11 and C7 - c.35V DC
• R11 (which is a link) - c.20v DC
• Junction of R2 and Q1 collector - c.7.5v DC
• Across both R4 and R10 - between c.2v and c.4v DC, depending on the Vp cut off voltage of the specific FET used. Both values should be the same.

Using an oscilloscope with at least a 20MHz bandwidth, and a x10 probe with an impedance of c.1M and a capacitive loading of c.15pF -
(again reference to gnd) :
• Junction of R3 and T1 primary - between 4.5 and 5v AC p-p at 10MHz
• At each end of T1 secondary - between 3.5 and 4v AC p-p at 10MHz.
(those last 2 readings may be slightly different from each other, but typically less than 250mV difference )

• Measuring the signal at Q4 gate will not give a very accurate reading, because the 'scope lead will load T2 secondary, but will typically be a sine wave between 500mV and 2v AC p-p at 10MHz.

Can you confirm that you are getting readings close to those values? ...

So I want to add replace sdc with ldc(cardioid capsule with 2 cables)on my Sennheiser mkh 416 rf condenser shotgun microphone. Please let me know how to do this, biasing and modification. Happy to pay if somebody is interested. by the way my first post on the group. Just joined it. Thanks everybody. You are lovely people. With love from pakistan

 

[rogs]

So I want to add replace sdc with ldc (cardioid capsule with 2 cables) on my Sennheiser mkh 416 rf condenser shotgun microphone.

First question has to be -- why? Presumably you are intending just to use the electronics, and mount them in a new enclosure along with this LDC capsule?... (There's no way an LDC will fit into a 416 body!).
As I say, I fail to see why you would want to do this, but you do need to be aware of a couple of things....
• Firstly, you need to measure the capacitive value of the existing capsule. Unless your replacement capsule has a very similar value, there's no way you'll be able to retune the inductor cores effectively.
• Sennheiser RF mic capsules mostly have a low tension membrane, and tend to be much more sensitive than conventional LDC capsules.
Those are generally designed to have a significant DC applied across them, and tend to have a higher tension applied to the membrane.
As result, you're likely to find your re-built mic will be a lot less sensitive than the original -- and probably noisier as well.

As I say, I don't personally think this is a very good idea, and I think you'll be disappointed with the results you get.
If you are keen on trying out an RF mic with a LDC capsule, why not have ago at the project being discussed in this thread?.....
(Project notes here: www.amx.jp137.com )

 

[Khron]

I was just googling for alternate sources for these elusive RF inductors. On Spectrum's ebay listing page for their 5u3HH, they mention they're equivalent to the Toko 3334/3337, so i googled for those too, and came across this:

https://www.sv1afn.com/en/inductors-1/variable-inductor-10-mm-5-4-mh-equiv-to-toko-kank3334r.html
Might this be a valid, but more affordable, option for us EU members? Even though the shipping's not the cheapest.

In the mean time, also found this (although "only" in packs of 20pcs):

https://www.qrphamradiokits.com/parts-components/shielded-coils/

 

[rogs]

I was just googling for alternate sources for these elusive RF inductors. On Spectrum's ebay listing page for their 5u3HH, they mention they're equivalent to the Toko 3334/3337....

Sadly, I don't think Spectrum's suggestion is very accurate?...
The T1 inductor employed for this project is required to have a centre tapped winding. Neither the Toko 3334 or 3337 have that.
The 'primary' (secondary in this application) turns differential for the 3334 is 7 and 11, with 2 and 23 for the 3337.
As outlined in Baxandall's original paper, the 'bridge' needs to be almost balanced for effective operation, and a non-centre tapped winding would probably make that extremely difficult to achieve.
The only Toko 10K series coils I have found which have a suitable centre tapped winding are the models KACSK3892, KACSK3893 and KACSK3894.
The last of those I have tried out. It comes with an internal 82pF capacitor fitted, which - being ceramic - is fairly easy to remove by a quick twist of a screwdriver.
Sadly though, the inductive value is only around 2.6uH, and that makes it difficult to use effectively with LDC capsules with a capacitive value of less than around 120pF. Not many of those about!

The Royal Mail international shipping costs listed on Spectrum's Ebay page are very odd.... It's cheaper to post to the USA than it is to France (or Finland) ??
All part of post Brexit EU/UK 'sniping' at each other, I suspect ?..

Despite Spectrum's Ebay page suggesting their coils are from China, I'm pretty sure they actually come from India.....
Where to buy this kind of coil in small quantities from India, I've yet to discover....

 

[Khron]

Well, poop...

 

[Khron]

On a more positive note though, have you noticed any sort of difference between the perfboard and the pcb versions, in case you've built one of each with the same component values?

Just crossed my mind whether the groundplane in the oscillator / capsule area has any meaningful (or negative) effect, an/or whether a gap in it would do any good.

 

[rogs]

On a more positive note though, have you noticed any sort of difference between the perfboard and the pcb versions, in case you've built one of each with the same component values?

Just crossed my mind whether the groundplane in the oscillator / capsule area has any meaningful (or negative) effect, an/or whether a gap in it would do any good.

I only ever built perf board versions of the early ideas for the project.

Notes and schematics here:

http://www.jp137.com/lts/RF.AMX4.construction.details.pdf
http://www.jp137.com/lts/RF.AMX4.schematic.pdf
Once you had laid out the PCB, I was not likely to return to my difficult to build - and rather inelegant - solution for the inductor assembly

There were 3 important 'events' in the development of this project:
• The discovery of the Spectrum inductors, which were much more suited than the TOKO ones used in the early experiments.
• The creation of your PCB
• The decision to replace the diode demodulator with an infinite impedance detector.

Once I had constructed the first version using your PCB I never felt any need to look 'backwards' to the perf board, as it were.

The ground plane construction seems to work well. It was never likely to make much difference to the audio side of things, and most of the RF was either contained within the inductors assembly, or superimposed on the low impedance output of T2, and it's connection to the FET gate.
It's unusual to have the FET effectively 'grounded' in condenser mic schematics -- we're much more used to FET inputs being high -- or more likely VERY high - impedance.
The ground plane under the inductor bodies serves to virtually complete the grounded casing of the inductors, and the relatively short external RF paths to the capsule and the tuning caps probably have less effect on the system than the external connections to the capsule - and indeed the capsule itself.
Those are of course largely contained within the grounded head basket.

In reality, once the assembly is complete, the inductor tuning will act on the whole RF structure -- PCB ground plane locations, cable routes, stray capacitance -- so that you are effectively optimising the whole assembly by carefully tuning those inductor cores.

As we have discussed at length in this thread, there are a number of different parameters that can have significant effects on how well individual microphones perform. I'm sure I don't understand the subtleties of many of them.
For example, one of the early perf board mics performed way better than others I had built in a similar way. I never discovered quite why....

But the later mics - using your PCB and the infinite impedance detector - outperformed all those early models.

The variations in (apparently) identical Chinese capsules can still spring a few surprises though.....

 

[reega]

V.5 of the schematic draws less than 3mA from the phantom power supply in total, so even a relatively 'weak' supply should function OK.

Using a DMM you should be able to measure voltages within the following values at these points, relative to gnd (pin1):
• Junction of R11 and C7 - c.35V DC
• R11 (which is a link) - c.20v DC
• Junction of R2 and Q1 collector - c.7.5v DC
• Across both R4 and R10 - between c.2v and c.4v DC, depending on the Vp cut off voltage of the specific FET used. Both values should be the same.

Using an oscilloscope with at least a 20MHz bandwidth, and a x10 probe with an impedance of c.1M and a capacitive loading of c.15pF -
(again reference to gnd) :
• Junction of R3 and T1 primary - between 4.5 and 5v AC p-p at 10MHz
• At each end of T1 secondary - between 3.5 and 4v AC p-p at 10MHz.
(those last 2 readings may be slightly different from each other, but typically less than 250mV difference )

• Measuring the signal at Q4 gate will not give a very accurate reading, because the 'scope lead will load T2 secondary, but will typically be a sine wave between 500mV and 2v AC p-p at 10MHz.

Can you confirm that you are getting readings close to those values? ...

Thanks again for you guide points on what to check! Finally got J113 from mouser (image attached of an authentic one .. without the fake Fairchild logo).

Also had to switch my crystal to 12 Mhz for the 55 pF condenser I've got. A little noisy right now but will hold judgement until unit is fully in case etc.

 

[rogs]

Thanks again for you guide points on what to check! Finally got J113 from mouser (image attached of an authentic one .. without the fake Fairchild logo).

Also had to switch my crystal to 12 Mhz for the 55 pF condenser I've got. A little noisy right now but will hold judgement until unit is fully in case etc.

55pF is likely to be on the low side for optimum results....
I did try fitting a 12MHz crystal in some of my early experiments -- there's a mention of that in one or two early posts in this thread - but I found that the system 'Q' drops off quite lot using frequency that high with these particular inductors, and the noise level rises as a result.

The suggested range of 65pF to 90pF for capsule capacitance - run at 10MHz - seems to give best results.
If you're lucky enough to find a capsule with an even higher value - say above 100pF - then running the oscillator an 8MHz will give an even higher 'Q', with potentially even better results.
The hunt for a suitable alternative 'off the shelf' inductor that will work well with lower value capsule capacitance has - so far - been a rather elusive exercise... but we can live in hope!

 

[Grim Reaper]

So I want to add replace sdc with ldc(cardioid capsule with 2 cables)on my Sennheiser mkh 416 rf condenser shotgun microphone. Please let me know how to do this, biasing and modification. Happy to pay if somebody is interested. by the way my first post on the group. Just joined it. Thanks everybody. You are lovely people. With love from pakistan

Very bad idea. You would ruin an excellent microphone and destroy its value. Buy a microphone that does what you want/need or build a new one from other components.

 

[kingkorg]

Hi @rogs

I am done with my build, although i used
2n5551 until Bc549 arrives. Not sure if that will affect the noise much.

Have you noticed change in the voicing of the mic when you tune the inductors? If i go for highest output i get huge LF boost. Take a look at these measurements. They are all same 90pF capsule just different inductor settings.

 

[rogs]

The 2n5551 has a lower gain than the BC549....and has a higher noise figure. Not sure whther either of those charateristics will affect the result in this application?
The inclusion or R3 and the 'Q' of T1 tends to ensure the oscillator output is pretty linear, in my experience --- although that tends to be relying on the visual oscilloscope representation of the waveform, rather than being able to measure distortion accurately at 10MHz !

I haven't noticed any particular LF lift. I don't have the facility to measure LF response very accurately, and tend to rely on comparative readings.
I normally use my Rode NT1 (black version) as a reference, when doing that.

I don't know whether the fact that - in theory anyway - the LF limit of an RF mic is DC has any impact?
I realise thats not actually true in practice, but there maybe more LF than one might expect from a conventional LDC impedance converter? ...

I have encountered quite a few variations in my experiments using different capsules, oscillator frequencies and capsules....

There have been 2 types of Spectrum '5.3uH' coils....
• The earlier ones -- marked '5R3' - tended to work best with 90pF capsules when used with an 8MHz oscillator.
• The later (current) ones are marked '5u3H' and seem to work better with a 10MHz oscillator, even wth higher value capsules.
The value of C4 needs to be fairly close to the capsule value..

I have had examples where there are several 'peaks' within the inductor tuning range, although there always seem to be a dominant 'peak' which is not difficult to find.
If the value of the capsule and associated bridge capacitor is outside the useful range, one can encounter the dominant tuning peak where the inductor core slug is right up at one end of its range...... I've found it better to change oscillator frequency to bring the inductor slug position back within a more central location.... That seems to help with the system 'Q', and ultimately the noise figure.

There are a lot of variables within this simple circuit. I've not encountered anyone - so far - who has been able to rationalise the theoretical maths involved in the inductor assembly, so that we can work from a known 'status quo'.
It seems there are simply too many 'unknown variables', to allow 'sim' software to be very useful here, for example.

So practical experimentation seems to remain the best option at present.
This circuit can work very well... but getting it to do so predictably is still a bit of a 'work in progress' !

 

[kingkorg]

The 2n5551 has a lower gain than the BC549....and has a higher noise figure. Not sure whther either of those charateristics will affect the result in this application?
The inclusion or R3 and the 'Q' of T1 tends to ensure the oscillator output is pretty linear, in my experience --- although that tends to be relying on the visual oscilloscope representation of the waveform, rather than being able to measure distortion accurately at 10MHz !

I haven't noticed any particular LF lift. I don't have the facility to measure LF response very accurately, and tend to rely on comparative readings.
I normally use my Rode NT1 (black version) as a reference, when doing that.

I don't know whether the fact that - in theory anyway - the LF limit of an RF mic is DC has any impact?
I realise thats not actually true in practice, but there maybe more LF than one might expect from a conventional LDC impedance converter? ...

I have encountered quite a few variations in my experiments using different capsules, oscillator frequencies and capsules....

There have been 2 types of Spectrum '5.3uH' coils....
• The earlier ones -- marked '5R3' - tended to work best with 90pF capsules when used with an 8MHz oscillator.
• The later (current) ones are marked '5u3H' and seem to work better with a 10MHz oscillator, even wth higher value capsules.
The value of C4 needs to be fairly close to the capsule value..

I have had examples where there are several 'peaks' within the inductor tuning range, although there always seem to be a dominant 'peak' which is not difficult to find.
If the value of the capsule and associated bridge capacitor is outside the useful range, one can encounter the dominant tuning peak where the inductor core slug is right up at one end of its range...... I've found it better to change oscillator frequency to bring the inductor slug position back within a more central location.... That seems to help with the system 'Q', and ultimately the noise figure.

There are a lot of variables within this simple circuit. I've not encountered anyone - so far - who has been able to rationalise the theoretical maths involved in the inductor assembly, so that we can work from a known 'status quo'.
It seems there are simply too many 'unknown variables', to allow 'sim' software to be very useful here, for example.

So practical experimentation seems to remain the best option at present.
This circuit can work very well... but getting it to do so predictably is still a bit of a 'work in progress' !

Awesome project, and tinkerer's dream. I am curious also to measure the response of known capsules without DC applied to them and see if there's difference. Once i nail the circuit of course. Thanks as always!

 

[ricardo]

rogs, have you compared the noise of your best RF version to your black Rode NT1?

 

[rogs]

rogs, have you compared the noise of your best RF version to your black Rode NT1?

With the same preamp gain setting, the RF mic typically has an 'absolute' noise floore about 6 to 8dB worse than my Rode.
However my 'best' capsule is around 9dB more sensitive than the Rode, so requires less preamp gain to produce the same output level.
Subjectively therefore the 'effective' noise floor can end up marginally better than the Rode.
I realise this is not the way that actual published S/N figures are calculated, and need to be referenced to a calibrated signal of 94dB etc, etc.... but I don't have that facility here at present, and so can only do 'comparative' tests at the moment.

I have built around a dozen or so RF mics, and the noise figures have varied quite a lot.....
It seems that the noise floor is affected mostly by the level of signal present at the JFET gate. Whether it is directly proprotional to that level I've not yet confirmed?
The amount of 'bridge imbalance' will affect that gate signal level - and thus the noise output level for that particular mic.

My most successful versions have come when using low tension capsules. With cheap Chinese capusles, this is simply a lottery....
I did find 2 x edge terminated capsules which had a measured capacitance of around 80pF. They also didn't like more than around 50V DC applied, before they would collapse when used in a conventional JFET impedance converter configuration.
They were also more sensitive than your typical K.67 style Chinese capsule (by around 6dB).
I concluded therefore that they probably had low tension membranes ....

I also selected C4 to be close in value to the measured capsule capacitance, to keep the JFET gate RF signal level low.
The completed mic required significantly less preamp gain than my Rode - for a specific output signal - so the perceived noise floor was actually lower than the Rode -- ( even though of course the RF mic 'actual' noise output was slightly higher.)

I believe that Sennheiser's famous 'push pull' capsules have low tension membranes, and as a result can generate significant capacitance change for a given signal input. I would guess that is a significant factor in the MKH mics achieving their legendary low noise floor?

As I have reiterated througout this thread, my experiments were offered as one possible solution to Gerard's original request for a hobby RF mic --
This forum has some very capable members, so it maybe that others have - or will? - come up with alternative approaches to this fascinating concept.

I'm no academic -- very a much a 'try it and see' hobbyist.....
Improvements - and/or optimisations - of my simple circuit are almost certainly possible.
Whether any of our more experienced members have any interest is 'dabbling' in this fascinating branch of DIY mic building, we shall see?....

There seems to be lots of interest on this forum for a whole heap of Neumann style 'clone' projects --often complete with 'antique' power supplies! -- but not so much interest in RF mics, it would seem?.......

 

[ricardo]

With the same preamp gain setting, the RF mic typically has an 'absolute' noise floore about 6 to 8dB worse than my Rode.
However my 'best' capsule is around 9dB more sensitive than the Rode, so requires less preamp gain to produce the same output level.
Subjectively therefore the 'effective' noise floor can end up marginally better than the Rode.
I realise this is not the way that actual published S/N figures are calculated, and need to be referenced to a calibrated signal of 94dB etc, etc....

This is as good a method to measure noise as any. I do the same with my mike measurements on MicBuilders.

You put both mikes in front of some source and adjust gain so they both sound and/or measure the same loudness for that source.
Then without moving anything, you put both mikes in your quiet room/box and measure the noise.
If you have an accurate figure for the sensitivity of your ref mike, you can work out the noise in terms of spl.
The caveat is that different types of noise sound different and have different annoyance value; hence the different noise weightings ... A-wt, CCIR 468 bla bla
But just listening to the noise with the levels adjusted as above is good too. With a 'ref' as quiet as the Rode NT1, you'll have to record several seconds of 'noise' from each mike and listen to them with very high identical gain.

On Zephyr.doc on MicBuilders, I measure the comparative response & sensitivity to adjust the gain but can only guess at the absolute noise level (spl) as I don't have a reference calibrated for sensitivity.

On Zephyr.doc, I also check the electrical noise of the preamp at the gains used with different source resistors to see if the preamp is affecting these measurements.

But your results show your RF circuit with your best capsule is subjectively quieter than the black Rode NT1 ... which makes it very quiet indeed

 

[rogs]

This is as good a method to measure noise as any. I do the same with my mike measurements on MicBuilders......

Don't let Goran hear you say that! .......I got told off on Micbuilders for describing my noise level findings 'wrongly', but I've found it quite useful to do comparison readings, using the published spec of the Rode as a reference point.....
As the oscillator output seems to be the dominant noise source in my present RF schematic, I'm still looking for ways that it might be improved.
Even though the actual existing noise levels are essentially right 'down in the weeds' in most real world scenarios, I think it can still be useful to look at possible improvements... althought the 'law of diminishing returns' probably applies here.

I've found the Arienne Audio flat K.47 to be an excellent capsule, and have tried one out in a figure of 8 configuration (which is the 'natural' configuration for this particular RF mic circuit.)
It's not as sensitive as some of my other capsules, so the observed noise level is a little bit higher --- although it's still quite usable.

I'm thinking of building a second mic the same way. ...... That would give me the opportunity to try out a Mid/Side mic -- or maybe even a Blumlein pair configuration?
Using Mid/side or Blumlein pair mics with LDC capsules - outdoors - is not something that gets described very often....... (Most conventional Hi-Z LDC mics don't really like outdoors very much )
But these would be Lo-Z LDC condenser mics --- I wonder how they might work as wildlife recording mics?.....

A Mid/Side LDC high quality full range outdoor mic --- possibly a first? ! (Probaby not a lot of demand for it though ! )

 

[ricardo]

Using Mid/side or Blumlein pair mics with LDC capsules - outdoors - is not something that gets described very often....... (Most conventional Hi-Z LDC mics don't really like outdoors very much )
But these would be Lo-Z LDC condenser mics --- I wonder how they might work as wildlife recording mics?.....

The problem is a directional mike, Fig-8 or Cardioid, is VERY susceptible to WIND NOISE. It's an immutable law of nature. The extended LF of your low tension capsules will make this worse.

Not trying to put you off but recommend you think of serious windshields now instead of as an afterthought ... and heavy LF cut too ...

I know this from trying to use a Soundfield Mk4 (-3dB @27Hz) outdoors.

 

[Tubetec]

I was wondering about the type of inductor your using , its a sheilded type with an adjustable slug which forms the core , its more or less what you find in small old fashioned FM radios .
The kind used by Sennheiser are adjustable pot core types ,
The MKH416p48 uses an off the shelf SMD mount 1:1 transformer as L1 , and a pot core type as L2 , as far as I can see its the same kind used in the MKH range going right back to the earliest models .

Were mostly audio focused here , I cant help thinking someone well versed in RF techniques might be able to help us iron out the problems , someone used to aligning radio equipment, as thats exactly whats going on inside the MKH .

My own experiments so far with the 416P48 are jaw dropping , the directivity is like no mic Ive ever used , its like the off axis sound is very much reduced in level yet very very flat and neutral at the same time .
The noise from the mic you hear or see in REW is utterly dominated by environmental factors even in the quietest room . These mics are very often used out of doors , in which case a rycote blimp is manditory , As my MKH416 is missing its case and internal windscreen its very sensitive too plosives , Ive improvised a protective screen using a textile fibre mesh . In the case of the other smaller mics in the MKH range they seem to have an extra internal blast filter very close to the capsule , no doubt the equalisation curve takes this into account .


There seems to be some of the view that the Sennheiser MKH mic diaphragm is somehow floppy or loose , I can tell you by having looked at one thats not the case at all .

I was looking back at Kingkorgs description of the MKH80 capsule , the seperated back plates as used in the Elam 250 came to mind , if we needed a symbol to represent the MKH 80 capsule what would it look like ? is it five wires ? two diaphragms ,two back plates and mid point for everything ?

My understanding of the RF mic concept is theres much less pull on the capsule membrane than in a conventional DC biased scheme, anyone whos messed around with capsule voltages will be familiar with the sight of the membrane collapsing onto the backplate .
In the RF example maybe we only have to account for the sound causing deflection of the membrane , not a big offset in the position and probably a variation in capacitance like when DC is applied , it seems to me with RF a closer spacing between membrane and backplate is appropriate .

A tube based RF mic seems like a very nice idea indeed ,maybe the Stephens circuit could be adpated or modified ,sub-miniturised ?

I really cant overestimate the auditory awe factor playing around both the MKH mic ,my specially modified SSL preamp and a set of headphones , its like weapons grade listening listening equipment . Im definately getting into the nature sound recording field,


News reports from sonically hostile environments are posted all over the planet everday using MKH mics , when you see the weather guy getting half blown away by the storm its most likely an MKH in his hand . These things are made to live out of doors .

The original style foam windscreen is a giveaway to the mics useage , its fair to say the MKH's use is standard practise in broadcasting, film/tv production and classical music recording worldwide , but the studio recording gang like to hang onto their antique tube mics that sound like a bucket of bolts

 

[rogs]

I was wondering about the type of inductor your using , its a sheilded type with an adjustable slug which forms the core , its more or less what you find in small old fashioned FM radios .
The kind used by Sennheiser are adjustable pot core types ,
The MKH416p48 uses an off the shelf SMD mount 1:1 transformer as L1 , and a pot core type as L2 , as far as I can see its the same kind used in the MKH range going right back to the earliest models .

Were mostly audio focused here , I cant help thinking someone well versed in RF techniques might be able to help us iron out the problems , someone used to aligning radio equipment, as thats exactly whats going on inside the MKH .

Yes, the inductors are 'off the shelf' conventional radio IF transformers -- The idea of trying those came from Umsahanker, on a thread in the Micbuilders forum back in 2017.

The only previous hobby RF mic I've encountered was Uwe Beis' 2015 project, and it was the inductors that posed the main difficulties with that project --- especially as the original construction details are no longer available.

The Spectrum inductors work very well considering they weren't designed for this type of project .... but outside of the UK, shipping has now made them quite expensive.....
I'm currently looking at trying out contructing some similar inductors using blank 10k formers from Aliexpress: https://www.aliexpress.com/item/1005003604363316.html

It should be possible to both copy the Spectrum coils, and make others with alternative windings to experiment with different values.

I've not ventured into trying out the type of cores used by Sennehiser -- and it looks as if Rode use the same type of structure?

I suppose we migh get lucky and find a hobby mic builder in among the members who's also good at RF inductor design ----- although this thread has been around since 2019, and we haven't had any other alternative RF mic designs --- so far!

Regarding your comments about the tension of the Sennheiser capsules -- even though they don't actually ' flop about'- - they will have fairly low tension membranes - -and why not?-- they don't have to worry about 'collapsing' from high voltage DC being applied!

AFAIK, the MKH416 does not use the excellent 'symmetrical' capsules as used in the MKH 20 - 70 models.
But I think they use quite a low tension capsule in the 416?....

And as for outdoor use -- well that's what they were designed for really! .... RF mics - unlike normal condensers - are low impedance, so are not affected very much by changes in humidity for example.

Intersting to remember that Sennheiser first experimented with RF becasue they wanted to use bipolar transistors, rather than valves (tubes).
There were no FETs at the time.
Once FETs arrived in the early 60s, most other condenser mic manufacturers returned to Hi-Z condenser mic circuitry.
Only Sennheiser stuck with RF... They have now been joined by Rode, but that's it commercially, AFAIK?

So, a bit of a specialised field., with very few hobby projects - so far! As Kingkorg mentioned in another post .. a 'tinkerer's dream'!