One Man's Trash - RX Noise Bridge

Hi All,

I hope you're keeping well and enjoying your radio pursuits.  



I've been up to a bit more 'homebrewing' of radio related projects and this one was a doozer made up entirely from my collection of junk. Before I get into it I'll let you into a back story for why I've constructed it. (Yep, some of my 'waffle' and this time maybe rather long. )


When I was first licensed my very first foray into the lofty world of HF operations saw me joining an 80m Net (living dangerously?) with a quite shocking outcome. The 'chair' was very welcoming and I was soon passed around to other operators with one making a notable 'observation'.....

"Are you sure you're running 10 Watts?!!" came a voice in a rather indignant tone.

"Yes" I replied, then went into detail of how I repaired the Yaesu FT757gx I was using, had made a homebrew SWR/Power meter calibrated to a 50 ohm load along with a homebrew ATU and a tangle of wire sort of making up a shortish (3/8ths) dipole. (That little lot saw some time constructing before I could venture into HF and previously I had only used VHF/UHF.....with similar disdainful retorts.....apparently Baofengs are sh*t.  )

"But you're coming in rather strong?!" came the reply.

"Well, yes", I said after looking up on QRZ, "We are rather close".

"But I can hear you at Hack Green!" came another retort.

Rather sharpishly the 'chair' intervened and brought the proceedings back into more friendly territory but I soon made my excuses and left the Net. I didn't bother venturing back into HF SSB for some time after that and if anything it also prompted me to start up my digi-mode FT4/8 shenanigans.

On another occasion of trying some 40m SSB activity I faced similar unenthusiastic responses with one operator stating that "You should come back when you can use more than 10 Watts."

Sheesh, not exactly rewarding experiences and I'm pretty sure many wouldn't bother ever again with such awful examples of Ham 'friendliness'.

Luckily these are the rarer examples of knob-headed-ness to be found on-air and since that time there have been many exemplary operators who have welcomed, congratulated or enthused my radio efforts often with much patience to pick up my poxy 10 Watt signals and I thank them all immensely.  

Anyway, with the recent outcome of the Ofcom consultation it looks like the day may arrive when I can come back with more than 10 Watts.



Up to now I've been using my homebrew ATU mostly with FT4/8 and so have recorded the settings to give me a perfect tuning for those frequencies of interest. (With my latter 135ft doublet antenna squeezed into my loft/attic space it gives me everything from 160m to 6m operations.) You can see examples of how I achieved all this in my other postings of home made gear amongst this forum section. With that minimal setup I've managed to happily work the world with digi-modes and QRP power.

However, now that I want to venture back into the voice sections of the bands I'll need to tweak those ATU settings and I don't want to perform the indignant method of 'whistling-up' or smacking out a CW tone of an 'auto-tune' ATU over someone's QSO do I? (Many do unfortunately.)


Time to go back into the history books and build myself an 'antenna-noise-bridge'......





Nice looking little box of tricks of isn't it. (Luckily I found and cut down an aluminium extruded box I was given some time back - one of the bits of 'junk'.)

It isn't a full measuring bridge being tied to a 50 ohm termination (two 100 ohm resistors in parallel) which is exactly what I'm after to ensure my antenna/ATU impedance matches the 50 ohms my transceiver would like to see.

How does it work? (Rather splendidly if you must know and I'll give some basic details for those that have never come across one.)

The nice big switch (from some ancient control panel) is a toggle to make switch so you push it down to make electrical contact and if you let go it springs marvelously back to the off position with a satisfying clunk. This was cleverly thought out so that when I use one hand to push the switch down and my other hand twiddling the ATU I can't accidentally transmit it into and blow the damn circuitry to bits with RF power.

That nice switch actuates an internal relay to connect my RF line to the noise bridge circuitry and nicely lights up that little 3mm red LED (stripped from some bit of junk electronics) to let me know it's powered up.

Time for an internal view of the rather ugly electronic construction. (Ugly is not a problem if it all works!)



I suppose it's not all that bad and again every single part was sourced from my junk pile...I didn't buy a damn thing.  

Rather than a typical zener diode used for a white noise source circuit I used a reversed biased NPN transistor junction as it produces a far greater noise level. That way I don't need the typical multiple transistor amplifiers afterwards and instead was able to get away with a single transistor amplifier. Luckily I found some VHF transistors in my box of recycled components which should mean that it'll work all the way up to the 6m band at least.

This all is fed into that toroid you can see which makes up the bridge to feed RF noise into the transceiver. (The toroid was also a 'found' part and isn't particularly important so long as it's a ferrite material.)

So, when you flick that switch down the antenna and transceiver is fed through this box of tricks and the result is a loud S9+ of white noise hash on receive if the antenna/ATU is out from 50 ohms impedance. As you alter your ATU settings getting closer to 50 ohms the white noise signal level drops right down and when you find the lowest point you let go of the switch, the relay connects the antenna line directly back to the transceiver and you should be 'tuned' in. If you now check you're SWR reading it should be nice and low. (Usually 1:1 but I won't quibble with a SWR reading of two or less.) I've checked the connection 'losses' with a NanoVNA (cheers Alan) and it can't measure any so I'll assume they're quite low.


Now if this is such a 'fantastic' bit of kit how come not everyone uses one?

Well not everyone can 'tune' an ATU by hand for some reason and also if you have a high noise floor at your location/setup you might find it hard to distinguish the bridge white noise from the background hash. However I haven't found any of that an issue so it's really helpful to me.

Surely it takes ages to 'twiddle' the ATU even with this contraption? Not so....

Because I already know the band settings for the ATU (recorded due to my FT8 forays) I only need to 'tweak' the settings and usually only takes a second or two. Besides, it's a much more 'polite' method of tuning and worth the while.


Is all of this going to help me on-air with some voice fun? 

Should do and I'm looking forward to giving it a go with my current 10W as well as looking forward to what the proposed 25W limit should reward me with. Besides I have another antenna planned in the works that may help me out a bit and it won't be a scrunched up affair in the loft. (Surprisingly should fit into my small plot too and I'll be sure to post details once I get it up.)


Now I know some of you may think I'm bringing the Ham radio interest into disrepute with my moaning about some of the more 'eloquent' operators out there. This is not so, but we shouldn't just sit back quietly whilst certain individuals think they have the right to knock back new operators whenever they can. Luckily as mentioned there are some fantastic operators out there who are very welcoming to all involved and for me anyway, much more admirable than any list of expensive equipment or far flung QSO's will ever be.

It's as I always told my children and lucky to have grandchildren to tell them the same.....

"Do whatever you want to enjoy yourselves, so long as it isn't to the bereft of others."


My RX-Noise-Bridge should fit the bill nicely.


Apologies for the longer than usual waffle but surely it must make a change from the call sign request postings.

All the best,
Victor

26CT3228
M7VIC

Lol well done victor on the build. 
I sort of had the same response the other day, are you sure you running 10w,all they had to do was Google (other browsers are there) and see a ts120v only runs 10w...... No little switch, or dial to move 10w or nowt, I know I could put an amp with it but they cost money and being a northerner that will never happen lol
But like you there is still a few on Hf I have found, M7 not talking to him brigade, only running 10w

Cheers Jimmy mate.  

I still find knocking stuff like this together (despite my nowadays bat like eyesight and bear paw hands) a lot of fun.


Never let anyone knock you back at all there Jimmy and just enjoy what you're doing regardless. You do rather well with your radio antics and appear to thoroughly delight in it all. Long may it be so my friend.

73

roll of feb is all i can say m7vic ,atu`s for sale, recon rig man and god know what else you will be building i can see a big valve amp on the cards next, at least you would be warm.
dont bother me one bit if some dont get back its the ones that do that make it more fun esp when i call qrp and they get right back like the other week end on 10m, as soon as i said 10w qrp they got back mind it was a good day here for 10m booing in from every where the 25w would have helped i think but like you i like the thought of getting out with low power, even my cb is stll only 4w but dont do to bad, just right place right time hence nothing else on the challange as of yet but weekend soon be here and the skip rolling in i hope.
ft8 in the new, year funds wont let me get signal link yet as radio 35 years old and no input/output dont know why lol.....and also we can then run them remote woohoo,,,,,, but saying that how much work will i get done if im looking all the time lol........maby not

Great post and write-up, Victor. Top work on a fab project and even more amazing that you have constructed it from bits and pieces you had lying around. You have great knowledge and skills too.

Thanks for taking the time to post up your work.

73s John.

Cheers John, really do appreciate your kind words my friend.

I've already started tinkering/modifying it after seeing details of the old Heathkit HD-1422-A that has a tone function for easier operation. After digging through my bits I found a drawer of old 555 oscillators but better test them as the manufacturer date stamps are from 1982!   


All posted for a bit of enjoyment and maybe even help for fellow members.


All the best,
Victor

An excellent post Victor.  While this type of thing is beyond me, I hope to learn much more about the electronics side to radio use.  So thank you!

Al

looks very handy. I could do with building one of these for myself . It'd be very useful for designing band pass filters in conjunction with a spectrum analyser I've got. It's not got a tracking oscillator. Any chance of sharing the diagram? a quick sketch would do.
Regards Steve W (g8fgb)

Hi Steve,

I've got to admit that since being gifted a NanoVNA I use that for almost everything including sweeping filter circuits. (It's a handy bit of kit!)

"A quick sketch" is no problem, it's the photographing it on my phone, emailing it to myself, no doubt editing to white balance, re-scaling etc. that'll take the time. (Still quicker than most of the CAD programs out there!)

It's a simple enough 'school-boy' circuit but leave it with me and I'll sort out uploading a basic schematic.

All the best,
Victor

Right, here we go...schematic/circuit diagram :-



(Apologies for the terrible quality but quickly 'snapped' on my ancient iPhone6 from a quick hand drawn sketch.)

Reverse biased junction NPN transistor as the noise source which provides a higher output level than a simple straightforward zener diode would. Single transistor NPN amplifier follows and then off to the toroid winding. (Simpler zener circuits are usually followed by at least three transistor amplifiers.)

As mentioned in the text these were junk box items, the VHF transistors provided noise all the way up 6m (50MHz) so did the job for my setup. The resistors are not all that important and nearest values would do, same thing for the capacitors and I had 1nF so used them, 2n2, 3n3, 4n7, 5nF etc. should work just as well. The toroid was a junk box item but I've just measured it's dimensions and from what I can gather it'd be a FT50-43. A smaller FT37 would do just as well but my old fat arthritic fingers just about managed with the FT50. Also hence why I stopped at 8 trifilar turns but pretty sure it'd work just as well with 9, 10, 11, whatever.

I've marked the start of the windings to how I wired it up (red dots) but if you have any problems it may be worth swapping the connections of the transistor amplifier side. The two 100 ohm resistors simply provide the 50 ohm load required, (and easier to find than an actual 50 ohm resistor.)

I wouldn't think this type of circuit would provide you much in the way of output level but was good enough for the front end of my receiver.

I've since added a 555 astable circuit feeding the reverse bias junction transistor via the 100K resistor setup to provide a rather splendid 'farting' sound on receive. Satisfying to tune out the 'fart' and easier to distinguish from the background noise hash.

I haven't drawn the relay contacts I used but a simple DPDT affair can switch in/out the antenna/transceiver connections. Big warning....don't transmit into the b*gger, hence why I used a push to make toggle thereby tying up my hands (Noise bridge/ATU twiddling) as it will 'pop' should you do transmit into it.

As mentioned, nothing glorious (if you're so electronically inclined) just nice simple circuitry.  

Oh, before I go, no I didn't design a 'PCB', (you get asked that a lot!), and it's a simple 'A-B-C' circuit which if you've learned the language of electronics can make for some wonderful 'words'. You'd be amazed how people recognise the letters 'C' or 'B' of an electronic circuit and declare it as a 'copy' of something else. Of course it is, we stand of the shoulders of giants and simply learn the 'language' to fulfill our needs. (As well as what's laying in our junk pile!)


Hope this helps at least someone.

All the best,
Victor

Thanks for that. very useful.

now if only i can turn my microwave in to a linear burner id be happy,    nice project victor,