The HomeBrew Workshop

Do you enjoy making your own equipment or antennas? Discuss construction and design in here.
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LastTemplar
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The HomeBrew Workshop

Post by LastTemplar »

Thought i'd start a post about homebrew test equipment, feel free to add your projects.

All i ask is for detail on what it does, how to make it and what parts you need...

So here is my little attempt, something i needed and hopefully it will be of some use to anyone new to Cb Repairs, Mods Etc.

So this is a simple Rf Sampler.

What it does is Sample a small part of the Rf signal, small enough that it doesn't cause damage to delicate test equipment ( you don't want to connect the Rf output of the radio directly to a Scope or Frequency counter of the bench kind)

So you looked in a service manual for your radio and it had a diagram showing a basic connection layout of scopes, frequency counters and so on. Did it make sense or did it leave you questioning how?

Here is a diagram which may help.
Image

Ok so right off the bat we see the connections the manual wishes us to make are via the probes or there inputs. It also shows this Rf sampler? well the service manual doesn't show that, where do i get one and how much is it.
Cost wise a shop bought item isn't cheap... Luckily you can make one for a few pennies to a few pounds. This depends on which type you make and what parts you have already. it also depends on how far you take the design.

So for my build i looked at all the different designs, i decide to go look see what i had laying about and construct accordingly.

So i had this laying around, a vintage antenna tuner (well its crap) inside are a few coils of bare wire 2 connecting loops and some iffy looking ? Reistors or capacitors by turning the knobs more or less contact is made and this must "TUNE" the antenna, well it might fake a restistance i guess.

Image

Well its not a bad project box, it also has 2 panel mount So239 sockets ideal

the guts were removed along with the stickers, leaving a box, 2 small holes and 2 So239 sockets.

Image

Next i found a ferrite ring, in fact i found several. Green ones and yellow ones, these were on the mother board of a defunct xbox 360 but many boards have them aswell as power supplies (think PC)
i chose a green one as the center hole is a little bigger ( these come with enamal coated wire already on them) i removed it and opted for a thinner wire which was an Rf coil (i have a bunch i will never use)

i cut off the original wire and re wrapped the core with about 6 full loops, evenly spead about the core.
I slipped the core over a short length of rg58 coax.
Strip back the ends of the coax as if fitting a plug.

Solder the inner conducter of the coax to the center pin of both the 239 sockets in the box
Solder the braided coax to the outer edge of 1 pl259 socket this is the input socket. Where you radio will connect.
Now you need a panel mount BNC connector (ebay, Maplins) about quid maybe 2. i use the crimp on type as a rule, the one on this poject is one for panel mounting but is designed for coax this was just what i had to hand.

solder resistor to the center pin of the BNC connector approx 100 ohms i used a 120 ohm the value isn't overly critical somewhere in this area will be fine. NOTE too high a resisitance (a 1k ohm may not work) and it wont pass enough signal too low and it may pass too much.

Image

drill out one of or a hole to fit the BNC typically 13mm.
mount the socket and solder one end of the small wire wrapped around the ferrite core to the resistor as this type of socket is designed to go on coax i slide an off cut of the clear insulator from the coax center over the leg of the resisor so it wont short to the outer connector body.

Image

The other end of the ferrite core wire need to be soldered too the earth point where the first braid connection was soldered to the radio input connector

Image

The final connection is the loose end of the coax braid as you can see in the above picture that needs to be soldered too the outer shell of the BNC connector.

The Rf Sampler is now complete.

Connect the input too your test Cb
The other Connector (pl259) goes too your dummy load or antenna
The BNC connects to your test gear

The small audio socket i fitted to tidy up the hole and later will be a a modulation output connector.

a note on BNC connectors, the crimp ones i use are easy you can crimp them using a special tool or simply solder them on. tin the center coax lightly then slide the pin on heat up and leave too cool . The outer braid has a collar ovet it pinching it too the connector tail.
i tin the braid lightly then heat it too the connector tail. i then slip the crimp collar over and hopefully it fits.

if not no worries stand the BNC on somthing that wont burn or mark with the collar upwards. press down on the collar with some plyers heat the collar and when the solder melts the collar wil both slip nicely into place and solder on allow too cool for a moment and remove the plyers.

remember the connector will be hot for a while.
This here is the Crusader
Wishing you all the good numbers
Coz i got a pound in my pocket going off like a rocket
My Wheels are smoking and i'm not joking..

10-10 til we do it again.....

we down ...we gone
Mattylad
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Re: The HomeBrew Workshop

Post by Mattylad »

What an excellent idea for reusing one of those horrid little matchers.

I have one too - maybe I can do the same - although I was thinking of doing this to my ATU wich is always inline anyway, so I can have an output to a scope as a mod check.

However - whilst you have continuity between centre pins of the SO239's you have the braid of that coax outer going to the BNC and NOT the SO239 also. This then relies on the **** paint job of the enclosure to make that connection.
Would it not have been better to link the braid to the SO239 also?
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cjay
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Re: The HomeBrew Workshop

Post by cjay »

I think perhaps removing some of the paint and using serrated washers on the SO-239n and BNC connectors would be useful too, it'd then make good RF contact with the case.
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LastTemplar
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Re: The HomeBrew Workshop

Post by LastTemplar »

Matty...

There are many ways to sample Rf signals, for example if you have an antenna switcher in your shack with unused channels you can often pick up enough Rf signal from one of the unused positions. This method works on many but not all switchers units just remember never to switch onto the position you have your scope plugged into.

The design i have used here was an experiment and this is the configuration which worked the best, all the sockets are grounded via the case but looping the coax braid still gave a cleaner signal when set directly too the BNC at one end SO239 at the other.

In answer to your question, the centre feed must be connected pin to pin as you would expect, as a general rule this type of design using a ferrite ring as an inductor one end of the braid is earthed at the input connector and the other end is left open (no connection) and has extra capacitors.

You can use a simple resistive circuit to sample with out the need of the coax or a capacitance circuit as you like. Another simple method is to use a T piece and a male connector with the pin removed and a brass screw in its place adjusting the screw until you get a reading without it making contact with the centre pin of the T piece.

You can make a simple circuit using
1 SO239
1 BNC connector
1 5pf Capacitor 1kv
1 50ohm resistor 1/4 watt
2 diodes 1n914
This here is the Crusader
Wishing you all the good numbers
Coz i got a pound in my pocket going off like a rocket
My Wheels are smoking and i'm not joking..

10-10 til we do it again.....

we down ...we gone
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Andy
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Re: The HomeBrew Workshop

Post by Andy »

I use a homebrew resistive sampler. It's built on a slab of PCB with N-type in/out (load) connectors and a BNC sample out socket. The connection from 'in' to' load' is direct, the load being an external Bird 50r dummy load. The sample is taken via a resistive attenuator to the BNC socket. Output (sample) impedance is 50r to match properly into my spectrum analyzer The sample is 40dB down on the input and it's flat from DC to 1300 MHz - ish. I've got a plot of it somewhere....
40dB sampler.jpg
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'SOMEONE GET ME A SAW!'
Andy.
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