Hand Held Radios (ICOM)

[chook]

Hi Ross, I just spotted this as I have been at the airfield for 3 days.
Ok we aim for a perfect Standing Wave Ratio (SWR) of 1:1. This transfers all of our generated Radio Frequency (RF) power up the antenna and radiated out so that receivers can pick it up. It is very rare that we ever get a SWR of 1:1 over the whole RF band for a particular antenna which is what the traces are showing here (its been some time since I used a Sitemaster analyser) but essentially we want a flat line as close to the bottom of the screen as we can.
In the first pic the left hand vertical line (graticule) is showing a SWR range between 1:1.24 and 1:3.11 with an acceptable SWR for the particular antenna under test at about 120 - 125mHz (the bottom graticule showing frequency). Radio manufacturers like to have their radios working into an antenna with a SWR better than 1:1.5 so this antenna will satisfy that requirement between about 117 and 132mHz with a larger loss in the 128mHz area.
Unfortunately they don't show in the pics where this particular antenna is mounted during these tests as mounting it onto your gyro can improve or make the results worse. testing the SWR in a hanger is also problematic and will give you erroneous results. So the job of the radio tech is to determine where on your gyro is the best location giving you the best SWR value with the least amount of signal shielding. For example mounting the antenna on the front of your gyro say at your instrument pod will give an excellent signal forward but if the receiving station is to your rear and below you then both you and your engine are absorbing much of the RF in that direction. The spinning rotor above us is also problematic as it acts as a giant revolving ground plane that affects signals upward. Honestly rotary wing aircraft are the most difficult aircraft to get radios working eficiently but with patience and knowledge we can get them almost right!

Regards......Chook.

[RossM]

Thanks Chook.
this was done at a shop in town. I used the fellas SWR meter outside the shop and hand held the antenna.
I had asked Rojone to make it centred on 126MHz

[blakemorem]

Hey Ross, i work with anritsu site masters nearly every day and they are very accurate and save you a lot of time if you believe what they are telling ya, because they're never wrong... just one thing i noticed is your cal is off so your reading will be Bullsh**t. Let it it warm up then calibrate it, then do test, it can make a lot of difference to your test.
Mike

[RossM]

That is why I posted the image, was not sure how accurate the reading was.
How do you calibrate them?
Regards...Ross

[blakemorem]

Hi Ross, The functions of the Anritsu may be little bit complicated but is a very useful machine for fault finding in cellular networks, So I'll do my best and try to keep it as simple as i can because the RF theories involved are interesting but complicated.I think the mode you displayed is VSWR Voltage standing wave ratio, Similar to spectrum analyzing, Its measuring the RF bi directional.Its signal generating preset band RF output and measuring reflected RF and calculating algorithmic loss at frequency in decibels or voltage.
10 log P2/P1 from memory. The thing to keep in mind is it's a logarithmic ratio Input/Output /and loss. So first thing is to give it some parameters to work with. Select mode to do the measurement you want. Set frequency F1 to the lowest band width and set F2 to highest band freq. There are a few functions. DTF= Distance to fault. CL=Cable loss,RL= return loss SWR. WARNING One thing i need to tell you here Ross is with these modes the radio should be off and antenna line disconnected, if you inject RF into the Anritsu or especially the CAL-T called the dummy load you could do damage and the CAL-T is very precision and VERY EXPENSIVE.To calibrate let it warm up about 10 minutes select mode Say DTF, Select DTF AID Select cable type and set F1 and F2. Then press Start Cal, It will tell you 1. connect open 2.Connect Short 3 Connect load. WAIT during these steps as its measuring and calculating,then it should say cal successful or CAL ON and you should have a nice flat line on your trace, meaning its cal to end of your fly lead is Zero distance/loss at end point. Now you can connect to your antenna line and determine the RL/ Return loss DTF/ distance to fault and CL/ cable loss. Usually we have adapters in the kit to connect dummy load to cable end during these tests. you can set D1 and D2 pending on length. Use the marker to peak in DTF mode to determine end of cable with 50 ohm dummy load on end of cable.The cal T has OPEN SHORT LOAD marked on it and usually easy to identify Ya see how easy it is to damage your cal T as you are using one connection for testing and if radio is connected and transmits ya cal t is stuffed and lots of money smoked. Usually problems encountered with antennas is cable kinks, bends, water ingress, and shorts/ impedance miss match. Our RL limit is -1.04 dB, so it has to be almost perfect and can be difficult to achieve = crap day on the job.
Anyway have fun Ross,hope this helps a bit and i haven't confused ya too much.
Mike.

[chook]

Thanks Mike - it was all familiar but deeply buried in my tiny brain. It's a bit like algebra - we use that every day don't we?
I'll bet Ross want ask that question again.

Regards..........Chook.

[RossM]

I will just print this out and show Yogi bear (fella who owns the meter)

[blakemorem]

Hi Ross, yeah they can be a headache, but the simplest way is try to select Function then cable type, as there is a default list in the DTF AID menu, Then select Start Cal and follow the prompts, When finished Press SAVE CAL and edit Cable type of Test Name.If you navigate the menus you will get used to it in time. You can also save the trace and edit name as well for reference say if you think you have fixed fault or fault reference. It's quite a useful unit. Lets say you have a damaged cable. 1 select DTF RL 2. Select DTF AID and set cable type/ F1 /F2 and distance of cable average is going to be about 5m, 3, Start cal and save cal(nice flat line at bottom of plot) 4. put dummy load on end of cable and connect site master to other end, anything below 16 db is ordinary. Return loss similar procedure and remember unit is already set so just hit return loss and limit should be set to 1.06dB and watch trace should be under the line. in the old days you had a SWAR meter in line and you would trim the wire antenna until you had pretty much zero vswr. The problem these days is they are factory made to suit and average system. If you want a truly real tuned system the antenna needs to be tuned to your setup as every system will have a unique impedance and that is a product of variables eg Radio amp impedance/cable impedance/connector impedance/antenna impedance. The theory is that every circuit has capacative and inductive reactance for a given frequency. and Z impedance is a product of these two factors. When the capacative and inductive reactance councils out you achive whats called Q resonant circuit or perfect conductance without any reflections. So as you change frequency on your radio it all changes. Generally the cable is pre selectel for a given frequency bandwidth and generally the impedance is about 50 Ohms.The antenna is another parameter that has a predetermined wavelength for a given frequency band. All this stuff can be calculated and you get the finished product but its hard to engineer all the variables out and that's why you get average product,sort of covers a range of variables. If you want the real deal you need to tune your unique system. I had a Marconi bench lab years ago and i had all the interstate truckies lined up outside the door in the mornings as the word got around and i was simply tuning the output stage of radio to Q RESONANT FREQ, Then i had ACMA inspector rock up with testing license no big deal all good license issued and had interesting chat as i was TX 150km without trying and that's just it, if your circuit is tuned your radio should perform very well.
Mike.