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Old Dec 22, 2011, 12:59 PM
Engineer for Christ
IBCrazy's Avatar
Amherst, VA
Joined Jun 2006
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The first coupler is good for 900MHz-1.3GHz. It will work well for you.

-Alex
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Old Dec 29, 2011, 02:00 AM
The Dreamer
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Sydney, Australia
Joined Dec 2010
871 Posts
Quote:
Originally Posted by IBCrazy View Post
^That should work fine. I used an Agligent HSMS-286K. This stuff is very hard to spec out, sadly...

-Alex
Hi Group,

Agilent diode is available on ebay

http://www.ebay.com.au/itm/ws/eBayIS...E:L:OC:AU:1123

Data Sheet

http://www.avagotech.com/docs/AV02-1388EN

I have made up this IBCrazy diode detector directly on the flat rear surface of a SMA Socket PCB mount using direct connection for the SMD schottky diode, SMD 1k Ohm and 10nF ceramic cap. That is, the SMD components are directly connected hence shortest possible lead length.

This schottky diode is very sensitive and works fine at 5.8GHz. I am using it with a 200mW Vtx, 6dB in-line attenuator and 10db directional coupler.

Directional coupler used at 5.8GHz purchased off ebay 4 -12.4GHz 10dB coupling factor.

The reason for the 6dB in-line SMA attenuator connected at the output of the Vtx is to prevent possible damage to the Vtx in the event the antenna under test is disconnected at the same time the Vtx is still powered on.

Beware, this diode is extremely small in size and not easy to solder onto the pins. I cut off the unused pins and left only the the one diode with its pins. Actually I cut off (sheered off ) the pins with a hobby knife as the pins are too small for wire side cutters.

Hope this is of some interest to the group.

Lindsay
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Last edited by Pisces; Dec 29, 2011 at 02:04 AM. Reason: Corrections typos
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Old Jan 01, 2012, 08:16 PM
The Dreamer
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Sydney, Australia
Joined Dec 2010
871 Posts
IBCrazy SWR Meter..Analog meter direct reading

Hi Group,

Some may be interested in making an IBCrazy direct reading VSWR meter but with an analog meter vis using a digital multimeter and then doing the math to calculate the VSWR of an antenna.

Some time ago I purchased on ebay an old style analog multimeter with 50 uA 0.1V FSD 2000 Ohm coil resistance meter movement for doing VSWR measurements. This meter is still for sale on ebay at a low price. This cheap meter is a reasonable quality meter movement and accurate when tested against my Fluke 73 digital multimeter.

http://www.ebay.com.au/itm/19-Range-...item4158919dc8

Apologies first; for the quality of the attached photo, my camera is set to 640 x 480 pixel and the photo are to not good clarity..sorry about that..
Edit: Now with camera set to HD. Still not good but now more readable.

Picture 1: Paper VSWR scale using tape onto the meter face. This scale is a file on the amateur site and can be printed out on paper, cut out and attached to the analog meter face.
http://vk5ajl.com/projects/SWRMeter.php

Picture 2: Analog VSWR measurement step 1, disconnect the antenna from the coupler and set the VSWR to infinite using the pot.

WARNING: the in-line SMA 6dB attenuator must be connected to the Vtx output at all times when the Vtx is powered on. Otherwise there is the possibility of damage to the Vtx.

Picture 3: Analog VSWR measurement Step 2, connect the antenna and the VSWR can be directly read on the meter.

If anyone is interested I have a couple of other photo of some tests I did on the VSWR Meter for linearity and accuracy when using SMA attenuator for VSWR 1:1.3 and 1:1.15. Also the Schottky diode turn on voltage and diode non-linearity below VSWR 1:1.15 becomes evident and affects the dynamic range of measurement.

++ ok my DIY IBCrazy 5.8GHz clover leaf antenna is not well tuned..shows around a VSWR of 1:1.15. Usable but really could do with some tweaking.

Lindsay
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Last edited by Pisces; Jan 02, 2012 at 01:01 AM. Reason: Better picture attachment
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Old Jan 01, 2012, 09:29 PM
Good Better Best quest.
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Australia, VIC, Cranbourne East
Joined Apr 2004
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Tuning coax lengths with a SWR meter anyone ?.
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Old Jan 03, 2012, 09:08 AM
Engineer for Christ
IBCrazy's Avatar
Amherst, VA
Joined Jun 2006
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Quote:
Originally Posted by Pisces View Post
++ ok my DIY IBCrazy 5.8GHz clover leaf antenna is not well tuned..shows around a VSWR of 1:1.15. Usable but really could do with some tweaking.
Please tell me you are joking. An SWR of 1.15:1 with a DIY 5.8GHz antenna is nothing short of exceptional.

On the other hand, I might need to copy your setup for a meter. I like how you made this

-Alex
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Old Jan 03, 2012, 09:29 AM
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Romania, Dolj, Craiova
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Wouldn't been easier to replace the antenna with a 50 ohm load, and adjust the SWR value 1, instead using the 6dB attenuator?
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Old Jan 03, 2012, 11:27 AM
Always soldering
Joined Dec 2011
234 Posts
Hey guys its ok if I use a 2 or 3 dB attenuator?
I only found those spec. near to my location and at a reasonable price.

Thanks in advance!

Cheers

Martin
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Old Jan 03, 2012, 02:46 PM
The Dreamer
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Sydney, Australia
Joined Dec 2010
871 Posts
Quote:
Originally Posted by RENATOA View Post
Wouldn't been easier to replace the antenna with a 50 ohm load, and adjust the SWR value 1, instead using the 6dB attenuator?
Hi Renatoa.

There are a number of links to other DIY VSWR meters that have been posted in this thread. They all set the reference infinite VSWR on the meter scale by using an open circuit load and then adjusting the meter sensitivity POT to the SET point on the scale. The SET point is the infinite VSWR reference. That is disconnect the load (antenna under test) and SET the reference. Then connect the load under test and read the VSWR.

Simple in-line amateur radio and CB etc, VSWR meters have a pot for setting the VSWR scale to the reference point for 100% reflected power. VSWR = infinity

The method I have described for measuring VSWR using a directional coupler is the normal procedure.

Disconnect the load ( antenna) from the coupler and 100% of power is reflected back to the source ( transmitter output). This is the reference and used to set the infinite VSWR on the meter scale.

Most if not all Amateur radio transmitters have output power control. So the VSWR measurement procedure is to reduce the transmitter RF output power to a safe low power level so that when the antenna is disconnected the high VSWR ( infinite 100% reflected power ) back to the transmitter output does not cause any damage the transmitter output amplifier stage.

Most Vtx have a warning in their instructions: DO NOT OPERATE THIS VTX WITHOUT THE ANTENNA CONNECTED. Reason is that high VSWR may damage the the TX output amplifier.

FPV Vtx do not have output power control. Hence the need to connect the 6dB attenuator to the Vtx output. This protects the Vtx output from high VSWR when the antenna is disconnected from the coupler. Also if the coupler itself is disconnected the 6dB attenuator will protect the Vtx from damage.

Hope this explanation of the procedure helps

EDIT: Before I'm corrected I need to point out this explanation applies to single sided directional coupler used as a VSWR meter. On the other hand, nearly all commercially available lower frequency HF, VHF, UHF VSWR meters use a bi-directional stripline coupler. In this case there is a switch to transfer the analog meter from the forward power port to the reverse power port of the bi-directional coupler. Set reference (infinite) VSWR is then taken from the forward power port. Maybe Im still exposed to some correction here. Any comment would be welcome.

Lindsay
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Last edited by Pisces; Jan 03, 2012 at 09:53 PM. Reason: Applies to single sided directional coupler
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Old Jan 03, 2012, 03:10 PM
The Dreamer
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Sydney, Australia
Joined Dec 2010
871 Posts
Quote:
Originally Posted by IBCrazy View Post
Please tell me you are joking. An SWR of 1.15:1 with a DIY 5.8GHz antenna is nothing short of exceptional.

On the other hand, I might need to copy your setup for a meter. I like how you made this

-Alex
Hi Alex,

I must say thank you for your great work in developing a simple cheap VSWR meter that will work up to 5.8GHz. As you know i have asked some questions over on FPV LAB on your VSWR meter project build. The problem I had was my Schottky diode detectors worked fine up to 2.4GHz but at 5.8GHz the results were poor. Poor sensitivity and linearity and not meaningful VSWR readings.

In particular it was your Agilent schottky diode HSMS 286x that did the trick for me to get the good results from the VSWR directional coupler at 5.8GHz. This diode is specifically designed to have exceptional sensitivity for making direct detect receivers for RFID tags ( 2.4GHz and 5.8GHz ) down to a sensitivity of -55dBm when using DC bias and -20dBm with zero DC bias.

Im actually thinking of adding a simple circuit to add DC bias to the detector diode. From the data sheet it indicates the bias needs to be in the range adjustable 3 to 20 uA. The detector will then hopefully be much more sensitive and the output linear.

Keep up the great work
Lindsay
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Last edited by Pisces; Jan 03, 2012 at 03:12 PM. Reason: typos
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Old Jan 03, 2012, 03:44 PM
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Nobert's Avatar
Tucson Avra Valley, Arizona, United States
Joined Jul 2002
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Lindsay

Great work. I've built up the SWR as you have. Looking forward to your biasing scheme.

Neal
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Old Jan 03, 2012, 04:47 PM
The Dreamer
Pisces's Avatar
Sydney, Australia
Joined Dec 2010
871 Posts
Quote:
Originally Posted by Martinba77 View Post
Hey guys its ok if I use a 2 or 3 dB attenuator?
I only found those spec. near to my location and at a reasonable price.

Thanks in advance!

Cheers

Martin
Hi Martin,

Here is some info for the group on suitable in-line attenuator,

Similar to the directional coupler the attenuator must be designed to work correctly for the frequency band to be used.

Typical SMA in-line coaxial attenuator are specified for a particular microwave frequency range. Common frequency ranges are DC to 4GHz, DC to 12GHz and DC to 18GHz. Note; a DC - 18GHz can be used at say 1GHz but not the other way around. That is a DC - 1 GHz attenuator cannot be used at 18 GHz.

The cost of the attenuator varies according to the frequency range covered. The higher the frequency range the more expensive the attenuator.

The value of the attenuator chosen is 6dB. This value will attenuate ( reduce ) to 1/2 voltage which also is 1/4 power. I purchased from Ebay used attenuator from China a lot of 4 x 6dB 1W DC - 4GHz for $16 free delivery. This was some time ago. Originally, 5.8GHz was not in-use for FPV Vtx. So I chose attenuator frequency range DC - 4GHz

The 6dB value is a compromise between protecting the Vtx output from high VSWR and also not reducing the power too low for the correct operation of the directional coupler and Schottky diode sensitivity.

So with my 4 x 6dB attenuator I can connect them together in series to make 6, 12, 18 and 24dB. This is useful for doing microwave antenna testing. Example, doing reduced Vtx power for Vtx range testing. 6dB is the value for each doubling the LOS distance.

I have found that these DC - 4GHz attenuator are still accurate 6 dB at 5.8GHz.

Then if cost is an issue the DC-4GHz attenuator will work ok at 5.8GHz. If one is choosey and cost is not an issue then DC - 12GHz or DC - 18 GHz are ideal.

This is the SMA attenuator I use. This is not from China and more that I paid. There are many SMA attenuator for sale on Ebay. I did not trawl through all the listed attenuator. This is an example only.

http://www.ebay.com/itm/M-A-RF-MICRO...item43ac227fb6

Lastly, SMA in-line attenuator are not directional. That is there is no input and output as such. The attenuator can be connected either way around in-line and it is still simply the same attenuator value.

EDIT: I forgot to mention the power rating of the attenuator. Typically SMA attenuator without heat sink are rated for either 1W or 2W. Be aware the attenuator needs to be able to absorb the RF power of the Vtx. So a 6dB DC-4GHz 1W attenuator can only only used at the output of the Vtx rated at max 1W or less.

Hope this rather long explan helps for choosing suitable SMA in-line attenuator.

Lindsay
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Last edited by Pisces; Jan 03, 2012 at 08:11 PM. Reason: typos as usual
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Old Jan 03, 2012, 07:07 PM
The Dreamer
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Sydney, Australia
Joined Dec 2010
871 Posts
Testing the VSWR Analog Meter 5.8GHz

Hi Group,

Whilst I'm on a roll, I thought I would post my pictures of the test method I used to check the operation and linearity of the IBCrazy VSWR meter. This set up is with the analog meter as the display, Vtx 200mW, coupler 10dB, IBCrazy detector.

As noted in the post above I have 4 x 6dB SMA in-line attenuator. 6dB is half voltage.

The analog multimeter is set to 50uA full scale range with series 100kOhm POT for adjusting reference= infinite VSWR

All the SMA attenuator shown in the attached pictures are 6dB. So please observe the position(s) of the attenuators in the pictures for the following test procedure.

Step 1. Disconnect the antenna. Power On the VTX. Note the single 6dB attenuator that is always connected to the Vtx output. Adjust the POT to the reference full scale on the meter to the infinite VSWR on the meter scale. Do not touch this POT again for these tests.

Step 2
Test 1. Add an additional 6dB attenuator at the input to the directional coupler. This then is 1/2 voltage and reads 1/2 scale on the meter. VSWR on the scale is 3.0. All good for the test.

Step 3
Test 2. Add another 6dB at the coupler input. Now additional 12dB connected. This then is 1/2 scale + further 1/2 scale = 1/4 scale on the meter and should be a VSWR indication of the 1.75. A small error as the paper VSWR scale shows VSWR of 1.5 for this test. This I suspect is the small error due to diode non-linearity. But may also be connector loss at 5.8GHz. But still this test confirms the operation of the VSWR meter is ok.

Step 4.
Test 3. Out of interest I connected a 6dB attenuator at the coupler output. Now the reflected power is attenuated 2 x 6dB once going out to the end of the attenuator and once back into the coupler, Then this is 12dB for the reflected power. This gives the same VSWR indication as test 2. The VSWR = 1.5 on the paper scale.

Note, if a 50 Ohm termination is connected to the coupler output then the analog meter reads zero deflection for VSWR 1.0 on the paper scale. This is as expected.

Also of interest I found that the isolated port on the coupler must have a very good 50 Ohm termination or the measured VSWR results are not accurate and give false readings.

I guess all this may be interest to the more technical followers of this thread. The VSWR meter is working well for me. Im happy. Thanks IBCrazy

Hope I haven't confused anyone
Lindsay
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Old Jan 04, 2012, 05:54 AM
Always soldering
Joined Dec 2011
234 Posts
Hey Lindsay thanks much for the reply and explanation!
The one I found is a 2dB DC to 8gHz ($15) and another one 6dB DC to 12.4gHz($25) and I use the same Tx as yours.(200mW)

I have another Tx and is a 10mW! For what i understand now with that output power is gone be difficult to measure right?

Anyway I buy the Agilent diode in the same Ebay place as you on 13 december and still doesn´t arrive

I´m a newbie and all this stuff its pretty dificult to me but Im really interesting.

Thanks so much for your patience guys and for do all this work for others to learn!

Cheers

Martin
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Old Jan 04, 2012, 07:57 AM
Engineer for Christ
IBCrazy's Avatar
Amherst, VA
Joined Jun 2006
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Quote:
Originally Posted by Pisces View Post
In particular it was your Agilent schottky diode HSMS 286x that did the trick for me to get the good results from the VSWR directional coupler at 5.8GHz. This diode is specifically designed to have exceptional sensitivity for making direct detect receivers for RFID tags ( 2.4GHz and 5.8GHz ) down to a sensitivity of -55dBm when using DC bias and -20dBm with zero DC bias.

Im actually thinking of adding a simple circuit to add DC bias to the detector diode. From the data sheet it indicates the bias needs to be in the range adjustable 3 to 20 uA. The detector will then hopefully be much more sensitive and the output linear.

Keep up the great work
Lindsay
Finding that diode was a stroke of luck, really. The real issue is judging the sensitivity is selecting the proper R and C values. Higher resistor values make the unit more sensitive, but throw off linearity as well as accuracy. Low R values make the circuit insensitive, but accurate. The capacitor just needs a very high ESR (equivalent series impedance).

-Alex
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Old Jan 04, 2012, 10:55 AM
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Worcester UK
Joined Nov 2004
761 Posts
Very interesting stuff chaps, love the old skool analogue swr meter, just one question:

Will a 6dB attenuator be safe to use with a 500mw vtx? or do i need a bigger attenuator for more powerful video transmitter?
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