Has anyone got a circuit diagram for DIY portable antenna SWR checker?

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3metrejim
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Has anyone got a circuit diagram for DIY portable antenna SWR checker?

Post by 3metrejim » Sat Sep 29, 2018 3:03 am

After my post on a 5/8 wave antenna it occurred to me that a portable antenna checker would be useful.

I've checked the net, but can only find DIY versions for HF. Wasn't thinking anything complicated, just a variable oscillator with an SWR bridge. Two adjustments, frequency and CAL (as on an ordinary SWR meter) with a switch for CAL/SWR.

If someone has a circuit for something like this it would be useful as a guide, otherwise I'll have to make my own which will take considerably longer. From what I can tell it only needs an output of around 30mW (into the bridge) to be useful, and cover only 87-108 MHz.

I think this would make a useful test instrument that can be built, rather than having to use a small rig, and SWR meter that is a pain to use if out testing an antenna in an open space. Should be considerably cheaper than having to buy a dedicated antenna analyser and would get the job done.

If anyone can help with this, I'm sure it would be appreciated by many on here, not just myself. If no one can help, then I'll post the circuit of what I come up with, but it may be a while (having to test and experiment).

Albert H
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Re: Has anyone got a circuit diagram for DIY portable antenna SWR checker?

Post by Albert H » Sat Sep 29, 2018 8:21 am

Take a look for an "Antenna Noise Bridge". I've used one for years. You just need the instrument and a receiver with a signal strength meter that works on the antenna frequency. The only tricky bit is winding the coil!
noise-bridge-vk3zzc.jpeg
It's easy to use and will show you both the resistive and the reactive parts of the antenna impedance - remember it can be a complex impedance made up of resistance and either capacitive or inductive reactance.
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3metrejim
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Re: Has anyone got a circuit diagram for DIY portable antenna SWR checker?

Post by 3metrejim » Sun Sep 30, 2018 3:40 pm

I had a look on the site that your diagram came from (http://users.monash.edu.au/~ralphk/noise-bridge.html). Says you might be lucky to get it to 50Mhz so,again only suitable for HF really...

So it's work in progress... Many faults to pick from (not used RF transistors, matching is probably too narrow band, etc....), but at least you can get a rough idea before building these days, thanks to LTSpice XVII.
work in progress.png
The bridge section may be a problem, as can't use diode detectors at this low power level. Maybe some amplification of the imbalance signal first??
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3metrejim
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Re: Has anyone got a circuit diagram for DIY portable antenna SWR checker?

Post by 3metrejim » Sun Sep 30, 2018 9:52 pm

My problem now is measurement of a low level VHF RF signal from zero to approx 600mV, with a minimum useful level of 10mV - The level is really too low for using ordinary silicon or schottky diodes. I'd like to stay away from the fragile microwave zero bias diodes. Seems the measurement part, is the more difficult part.

I was originally hoping for a VHF version of the 'tenna dipper' but with a meter instead of an LED. Looks like having to try out some real circuitry instead of relying on the simulator - it sometimes gives unexpected results that don't tally with some of my experiences for some reason, plus it doesn't have a model for a germanium diode (yet).

Albert H
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Re: Has anyone got a circuit diagram for DIY portable antenna SWR checker?

Post by Albert H » Mon Oct 01, 2018 2:39 am

Jim
I've made this bridge circuit work up on 2m (144 MHz) and beyond. You just have to use transistors with high Ft in the noise amplifier. I used BFR96 in the one I made for my own use. This gives a useful noise signal to well over 700 MHz. You don't need diode detection - you use a receiver with a signal strength meter that works on the frequency that you're building the aerial for. Listening for the "dip" is easy enough, but a meter makes it easier.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

3metrejim
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Re: Has anyone got a circuit diagram for DIY portable antenna SWR checker?

Post by 3metrejim » Mon Oct 01, 2018 6:17 pm

No disrespect to you Albert, I can see the advantages of the noise bridge as a piece of lab equipment.
I wanted something 'quick and (not so) dirty, that I can carry easily and only interested in an antenna match for an already approximately correct antenna. I also didn't want to be carrying a communications receiver about that has an external antenna input. I do realise that my 'gadget' will generate a carrier but it shouldn't be much worse than one of those MFJ things - I do have a spectrum analyser to check a real-world build with.

I was going to post the schematic but seems like I can no longer post attachments??? :nodg

Looks like a board problem, both .png and .jpg image files are returning as invalid.

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Re: Has anyone got a circuit diagram for DIY portable antenna SWR checker?

Post by Albert H » Tue Oct 02, 2018 2:34 am

Jim
The beauty of the Noise Bridge is that it's a "general" solution. You can make it work on practically any frequency you choose. Also - I have a very small battery-powered receiver (a Tecsun) with a field strength indication on it, so that's not a portability issue. The noise bridge itself is built into a small diecast box with a pot for the resistive element and a variable capacitor for the reactive. The Bridge is powered with the usual small 9V battery.

The whole kit is pocket-sized, and an earlier version - with a small Grundig receiver - was used in Belgrade for the aerials for B92, where covert operation was truly essential! The noise radiated by the aerial is hardly above the noise floor, so is insignificant. This allows the construction, installation and accurate calibration of aerials before a transmitter is ever connected to them!
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;)

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Re: Has anyone got a circuit diagram for DIY portable antenna SWR checker?

Post by 3metrejim » Tue Oct 02, 2018 12:09 pm

Ok Albert, you've swayed me into building a noise bridge after all.

Seems you can use a very cut down receiver with one (but it needs calibrating first); a crude amplified wavemeter (I know from experience that they don't need much of a signal to work). No need to worry about a radiated signal, and a lot less power drain than my design - which would drain a rechargable 9v in around an hour. I also have used a similar transformer design, so may as well use that for a noise bridge instead (once I locate the 2 hole ferrite bead I have somewhere :!: ). Can't see that 'bang on frequency' is required, as a usual antenna has at least a few MHz bandwidth before it becomes unusuitable.

The link for anyone interested is here https://www.radioexperimenter.us/rm-199 ... ector.html

Just needs the tuned circuit altering for band II.

Thanks for helping, and pointing out a superior design to the resistive bridge circuit, Albert.

For those struggling with the OCR'ed text on the above link, the original article can be found here https://ia800604.us.archive.org/12/item ... y_1996.pdf (courtesy of the internet archive) on page 78.

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