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Old 26th Nov 2017, 1:31 pm   #1
Wendymott
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Default Directional coupler DC - 300 Mhz

Hi Peeps. I bought a couple of wound components from Ebay... see photo
with additional 50 R SMD resistors.
Unfortunately the vendor did not supply any info re schematic etc. Have any members bought such an item/s and have any useful application data.

I have contacted the vendor but no reply as yet.
Your thoughts would be much appreciated.
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Old 26th Nov 2017, 1:43 pm   #2
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Default Re: Directional coupler DC - 300 Mhz

Are they not antenna matching coils for FM radios?
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Old 26th Nov 2017, 2:12 pm   #3
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Default Re: Directional coupler DC - 300 Mhz

Hi Wendy,

If you want to make a wide band largely frequency insensitive directional coupler that works well at VHF and above, its simple to avoid transformers and cores and simply use a short length of coax with some wires added into it, I used the thin teflon coated wire wrap style wire. Its just a matter of bunching up the braid and threading it through.

I've attached an article on a VHF directional Watt meter I designed once (when my callsign was ZL1WWV..always on time) using this method, probably the directional coupler design will be readable. I can't vouch for its low frequency performance, it probably would require more coax.

Anyhow, this method is something to consider when designing a directional coupler.

(If the images are not readable I could convert them to a pdf file and attach that)

Hugo
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Old 26th Nov 2017, 3:03 pm   #4
G8HQP Dave
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Default Re: Directional coupler DC - 300 Mhz

"DC" may be a slight exaggeration for a small wound component. 1-300MHz might be possible.
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Old 26th Nov 2017, 9:57 pm   #5
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Default Re: Directional coupler DC - 300 Mhz

It looks like a classic Sontheimer coupler (patented in 1966?)

The coupling factor is 20*log(N) where N is the turns ratio. So if there are 6 turns on the secondary and the primary just passes through the twin hole bead once then the coupling will be about 15.5dB.

These things often work over a decade or two of bandwidth although you usually only get good directivity over a decade or so. But a lot depends on the ferrite material used. Does your ferrite have any colour markings? eg a hint of purple for example?
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Old 26th Nov 2017, 11:29 pm   #6
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Default Re: Directional coupler DC - 300 Mhz

I've improved the drawing to show the mainline path and which port is the coupled port. See below. Often, the REFL port is just terminated in a 50R load and left alone.

I used to use a 10:1 turns ratio to get a 20dB coupler and used this a lot when repairing CB or ham radios. This allows the radio to be used into an antenna or a dummy load and the coupled port can be fed to a scope terminated in 50R. This then shows 1/10th the voltage on the scope.

You can also use this coupler to measure return loss and with a dual channel scope (or a vector voltmeter) you can measure the reflection coefficient (magnitude and angle) to then work out the complex impedance of a load. However, you do have to add a phase offset for this coupler type. Otherwise it gets the angle of the reflection coefficient wrong.
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Old 26th Nov 2017, 11:40 pm   #7
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Default Re: Directional coupler DC - 300 Mhz

Thanks Jeremy. I will make a pcb monday and come back to you.
To the other correspondents... Thanks for your input.
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Old 27th Nov 2017, 12:03 am   #8
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Default Re: Directional coupler DC - 300 Mhz

Make sure to be careful if you use it with your Rigol spectrum analyser. I wouldn't advise using it as a coupler to sniff at a (say) 4W radio because the power coupled into the analyser would be 36dBm - 15dB = +21dBm for a 6:1 coupler. 21dBm could be enough to damage your analyser. If the coupler develops an isolation fault then it could be even worse than this.

For coupling transmitters into an analyser I'd advise putting another large attenuator after the coupled port. Also, the coupler looks a bit small. It might only be OK up to 1 watt of power anyway. I used a larger ferrite for my CB/ham coupler and it was OK to about 25W PEP.

A 10:1 (20dB) version could be used with your Rigol analyser with its tracking generator to measure return loss over quite a wide bandwidth. However, I think a homebrew return loss bridge would work a lot better with your Rigol in terms of the directivity and bandwidth the return loss bridge would offer. But a tiny 15dB or 20dB coupler like the one in your image can be a useful tool to have on the workbench.
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Old 27th Nov 2017, 11:35 am   #9
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Arrow Re: Directional coupler DC - 300 Mhz

Quote:
Originally Posted by G0HZU_JMR View Post
Make sure to be careful if you use it with your Rigol spectrum analyzer. I wouldn't advise using it as a coupler to sniff at a (say) 4W radio because the power coupled into the analyzer would be 36dBm - 15dB = +21dBm for a 6:1 coupler. 21dBm could be enough to damage your analyzer. If the coupler develops an isolation fault then it could be even worse than this.
For coupling transmitters into an analyzer, I'd advise putting another large attenuator after the coupled port.
That is very sound advice. Several years ago when my employment was as a Test Engineer in the manufacturing industry of VHF broadcast transmission equipment, every engineer had a spectrum analyzer on his / her bench (Anritsu were the most common). We were rigourously instructed to always use a 20 dB attenuator on the analyzer's input port. The Test Dep't. also had access to a full-featured HP VNA: a superb item for aligning multi-pole LPFs. One day, it was discovered that someone had (presumably) decided to use it as a spectrum analyzer and had probably poked several tens of VHF watts into its input port. The repair bill was horrendous: management were furious, not unexpectedly!

That 'incident' aside, they were halcyon days. The work was absorbing, challenging . . . and reasonably well-paid. Unfortunately, the firm crashed during the 2006/7 recession. Early retirement inevitably followed.

Al.
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Old 27th Nov 2017, 11:46 am   #10
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Default Re: Directional coupler DC - 300 Mhz

Hi Jeremy and Al. I bought it specifically for the Rigol, for low power only. Thanks for the "heads up" though. I have had a reply from the vendor.... apparently its a "Minikits" thing.. so I will go to the Minikits website.
I have modified a pair of BNC adaptors for 10 and 20 DB attenuators...... Jeremy.. after your previous comments I looked on Youtube at various Rogol users.... and decided my kit was severely lacking and not using the Rogol to its full potential..... thus the sudden rush to kit up.
I also bought.. recently.. a number of torroids to make the higher power "return loss" bridge, but as yet, still in the stock to do box.
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Old 28th Nov 2017, 11:58 am   #11
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Default Re: Directional coupler DC - 300 Mhz

The best directional couplers that I've ever used are those made by Narda. Expensive when new, but second-hand prices can be quite reasonable, especially the ones for approx. 2 GHz and up.

Al.
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