mv031161
Well Known Member
Beware when buying a used Comm antenna. It works better if you purchased or procured from your hangar mate because of your personal relation but stay away when buying a used Comm Antenna from the open market unless it comes with a serviceable 8130 and in case of a Non TSO that it comes with some sort of guarantee and a test done with a SWR Meter.
Here is my story: I have had radio issues for the last 9 months. Depending on the day I was flying I was able to hear and transmit. however it became annoying. I have had the radio bench checked so I knew it was the antenna or the antenna coax.
The original builder had decided to build the aircraft using an internal Comm antenna. (The Archer Type for Composite aircraft). The original builder was not a cross country pilot and used the aircraft mostly over his aerodrome in New Zealand therefore his communications were limited to his home field radio a occasionally within 40 miles of it @ another field. IAW him, there was never a need to talk to ATC
Since I had an airshow trip involving cruising from Key West to Mexico all the way to El Salvador, I decided to buy a new antenna in order to avoid problems with Cuba and or any emergency. I was set to purchase and install a Ram 17-AV antenna when I asked my old hangar mate if he had one! Of course he had one and made me one of those offers that you sometimes just cant refuse! He said "Hey I got one, the one you help me remove from the S&F Tornado destroyed RV7. I will barter w you for some maintenance if needed on my RV8" Well......who can refuse such offer right? besides, I was already doing his yearly condition inspections.
I proceeded installing the used antenna with a grounding plate, pig tail, etc...all done accordingly to the manufacturer recommendations for a composite aircraft installation...ATIS/AWOS OK, Ground OK and Tower check OK. A quick test flight and all OK! People were hearing me and I was happy! Problem solved.................
A few days later I had to embark towards Central America for an airshow and my Comm problem issues started as soon as I left my field! in some Freq ATC could hear me and in some others could not! It was weird, I could hear everything from the lowest Hz Scale up to 121.00 and then from 130.00 to the upper limits but nothing in between!!!! As I was approaching Miami Center and about to enter class B airspace I lost all communication and decided to turn to tower @ KMTB for landing, got below the cake and proceeded to the airport. Somehow I managed to pull my trusty hand held and talk to KTMB tower for landing!
Now, i'm am at an airport I dont know anyone. it is Tuesday 4:45 pm and everyone is about to go home. Tower hears my broken communication and send me on request towards the local avionics shop. I ended up @ Peninsula Avionics at KTMB. I proceed by telling them about the antenna replacement and that's when they decide to install the SWR meter for test! Bingo! The antenna is faulty at some Freq. Only problem it is almost 6 pm and not a chance to get an antenna ordered for next day service any longer! Not even from California! Tech goes back to his shop and comes out with an old rusty, as removed RAM AV-7 antenna...same as the faulty one I have installed! He checked it with the SWR meter and it is good! I get my antenna installed and proceed to meet my travelling partners @ KEWY the next morning for our long ocean/country crossing all the way to Cozumel and El Salvador. I returned by myself and had great radio coverage throughout the route and all the way home.
To Peninsula Avionics...I got charged for 2 hours of work and nothing for the Antenna! what a great deal huh? these are all aviation people and getting the service I got from them was great! KUDOS to Peninsula aviation for saving my ***!
I hope my experience serves you well. Sorry about my English and or grammar! I'm not a US native!
Here is the procedure when checking and installing a new antenna.
Checking the SWR
Every antenna installation in an aircraft regardless of the quality of the antenna needs to be checked with a SWR meter before first flight.
Here is what you need to know, some basic facts:
· The scale on an SWR meter reads from 1 to infinity.
· The smaller the number, the better the SWR reading and the better you’re antenna will perform.
· The scale is not linear, it is logarithmic. From 1 to 3 covers more than half of the scale on a SWR meter. The remainder covers 3 to infinity!
· A SWR reading of more than 3 may be hazardous to your radio. (Often marked in red on the scale). Because of the logarithmic scale, you don't have to be far into the red before you are into the big numbers!
· For any given frequency there is a corresponding wavelength and one ideal length of antenna.
· Because we are changing frequencies often, it stands to reason that the actual length of our antenna has to be a compromise since we can't keep changing the length of the antenna every time we change frequency. The airband covers a very wide range from 118.000 MHz up to 136.975 MHz
· Most aircraft antennas are tuned in the middle of the airband frequency (usually 127 MHz).
· The ideal number to aim for on the SWR meter is 1:1 (pronounced 1 to 1) but this is rarelyachieved. This means all the energy is being radiated and none being 'reflected' back into the radio. A SWR of 2:1 or less is OK. Less than 1.5:1 is very good, more than 3:1 will damage most radios. It is generally regarded that a SWR reading of less than 2.5:1 is acceptable for aviation use but the intention is to get the SWR as low as possible.
· In practice the size and shape of the antenna's ground plane can affect the SWR of an antenna. (no ground plane = high SWR)
· ATC radio transmissions are vertically polarised. This means that your aircraft antenna should be more vertical than horizontal when mounted for best performance.
How to use the SWR meter
Connect the antenna BNC plug to the side of the SWR meter marked antenna.
Connect the radio to the side of the SWR meter marked transmitter.
In other words the SWR meter is connected between the radio and the antenna.
Rig the aircraft and move it well away from hangars (and hangers-on for that matter). SWR'ing inside a hanger is a complete waste of time because the meter will measure the signal bouncing off the metal from the hangar and give a high reading!
Firstly, set the first frequency to 118.000 and calibrate the SWR meter by doing a brief transmission* and adjust the power needle to the 'set' position (100% full scale deflection) using the knob on the front of the meter.
If you have the most common SWR meter which is a single dial meter you will have to switch between calibrate and SWR operations and note the readings.
*Note: You don't need to talk to anyone; in fact you don't even need your headset plugged in.
You are just measuring the efficiency of the 'carrier wave' not the modulation (voice signal).
Then repeat the operation above on 127.000 MHz and again on 136.975 MHz. This will give us three SWR readings across the entire aircraft band. One reading is at 118.000 MHz, the second reading at 127.000 MHz and a final reading at 136.975 MHz.
A typical example SWR reading follows…
118.000 = 2.3:1
127.000 = 1.5:1
136.975 = 2.8:1
From these three readings we can see that the antenna is best at 127 MHz, it is not too bad at 118 MHz but the SWR readings get high at 136 MHz. This tells you that the antenna is going to perform better in the central and lower frequencies than it is at the top of the frequency range. In
this example the antenna is not perfect especially at 136 MHz but it will do the job. Ideally we would like to get a better reading and to do this you tune your antenna.
Here is my story: I have had radio issues for the last 9 months. Depending on the day I was flying I was able to hear and transmit. however it became annoying. I have had the radio bench checked so I knew it was the antenna or the antenna coax.
The original builder had decided to build the aircraft using an internal Comm antenna. (The Archer Type for Composite aircraft). The original builder was not a cross country pilot and used the aircraft mostly over his aerodrome in New Zealand therefore his communications were limited to his home field radio a occasionally within 40 miles of it @ another field. IAW him, there was never a need to talk to ATC
Since I had an airshow trip involving cruising from Key West to Mexico all the way to El Salvador, I decided to buy a new antenna in order to avoid problems with Cuba and or any emergency. I was set to purchase and install a Ram 17-AV antenna when I asked my old hangar mate if he had one! Of course he had one and made me one of those offers that you sometimes just cant refuse! He said "Hey I got one, the one you help me remove from the S&F Tornado destroyed RV7. I will barter w you for some maintenance if needed on my RV8" Well......who can refuse such offer right? besides, I was already doing his yearly condition inspections.
I proceeded installing the used antenna with a grounding plate, pig tail, etc...all done accordingly to the manufacturer recommendations for a composite aircraft installation...ATIS/AWOS OK, Ground OK and Tower check OK. A quick test flight and all OK! People were hearing me and I was happy! Problem solved.................
A few days later I had to embark towards Central America for an airshow and my Comm problem issues started as soon as I left my field! in some Freq ATC could hear me and in some others could not! It was weird, I could hear everything from the lowest Hz Scale up to 121.00 and then from 130.00 to the upper limits but nothing in between!!!! As I was approaching Miami Center and about to enter class B airspace I lost all communication and decided to turn to tower @ KMTB for landing, got below the cake and proceeded to the airport. Somehow I managed to pull my trusty hand held and talk to KTMB tower for landing!
Now, i'm am at an airport I dont know anyone. it is Tuesday 4:45 pm and everyone is about to go home. Tower hears my broken communication and send me on request towards the local avionics shop. I ended up @ Peninsula Avionics at KTMB. I proceed by telling them about the antenna replacement and that's when they decide to install the SWR meter for test! Bingo! The antenna is faulty at some Freq. Only problem it is almost 6 pm and not a chance to get an antenna ordered for next day service any longer! Not even from California! Tech goes back to his shop and comes out with an old rusty, as removed RAM AV-7 antenna...same as the faulty one I have installed! He checked it with the SWR meter and it is good! I get my antenna installed and proceed to meet my travelling partners @ KEWY the next morning for our long ocean/country crossing all the way to Cozumel and El Salvador. I returned by myself and had great radio coverage throughout the route and all the way home.
To Peninsula Avionics...I got charged for 2 hours of work and nothing for the Antenna! what a great deal huh? these are all aviation people and getting the service I got from them was great! KUDOS to Peninsula aviation for saving my ***!
I hope my experience serves you well. Sorry about my English and or grammar! I'm not a US native!
Here is the procedure when checking and installing a new antenna.
Checking the SWR
Every antenna installation in an aircraft regardless of the quality of the antenna needs to be checked with a SWR meter before first flight.
Here is what you need to know, some basic facts:
· The scale on an SWR meter reads from 1 to infinity.
· The smaller the number, the better the SWR reading and the better you’re antenna will perform.
· The scale is not linear, it is logarithmic. From 1 to 3 covers more than half of the scale on a SWR meter. The remainder covers 3 to infinity!
· A SWR reading of more than 3 may be hazardous to your radio. (Often marked in red on the scale). Because of the logarithmic scale, you don't have to be far into the red before you are into the big numbers!
· For any given frequency there is a corresponding wavelength and one ideal length of antenna.
· Because we are changing frequencies often, it stands to reason that the actual length of our antenna has to be a compromise since we can't keep changing the length of the antenna every time we change frequency. The airband covers a very wide range from 118.000 MHz up to 136.975 MHz
· Most aircraft antennas are tuned in the middle of the airband frequency (usually 127 MHz).
· The ideal number to aim for on the SWR meter is 1:1 (pronounced 1 to 1) but this is rarelyachieved. This means all the energy is being radiated and none being 'reflected' back into the radio. A SWR of 2:1 or less is OK. Less than 1.5:1 is very good, more than 3:1 will damage most radios. It is generally regarded that a SWR reading of less than 2.5:1 is acceptable for aviation use but the intention is to get the SWR as low as possible.
· In practice the size and shape of the antenna's ground plane can affect the SWR of an antenna. (no ground plane = high SWR)
· ATC radio transmissions are vertically polarised. This means that your aircraft antenna should be more vertical than horizontal when mounted for best performance.
How to use the SWR meter
Connect the antenna BNC plug to the side of the SWR meter marked antenna.
Connect the radio to the side of the SWR meter marked transmitter.
In other words the SWR meter is connected between the radio and the antenna.
Rig the aircraft and move it well away from hangars (and hangers-on for that matter). SWR'ing inside a hanger is a complete waste of time because the meter will measure the signal bouncing off the metal from the hangar and give a high reading!
Firstly, set the first frequency to 118.000 and calibrate the SWR meter by doing a brief transmission* and adjust the power needle to the 'set' position (100% full scale deflection) using the knob on the front of the meter.
If you have the most common SWR meter which is a single dial meter you will have to switch between calibrate and SWR operations and note the readings.
*Note: You don't need to talk to anyone; in fact you don't even need your headset plugged in.
You are just measuring the efficiency of the 'carrier wave' not the modulation (voice signal).
Then repeat the operation above on 127.000 MHz and again on 136.975 MHz. This will give us three SWR readings across the entire aircraft band. One reading is at 118.000 MHz, the second reading at 127.000 MHz and a final reading at 136.975 MHz.
A typical example SWR reading follows…
118.000 = 2.3:1
127.000 = 1.5:1
136.975 = 2.8:1
From these three readings we can see that the antenna is best at 127 MHz, it is not too bad at 118 MHz but the SWR readings get high at 136 MHz. This tells you that the antenna is going to perform better in the central and lower frequencies than it is at the top of the frequency range. In
this example the antenna is not perfect especially at 136 MHz but it will do the job. Ideally we would like to get a better reading and to do this you tune your antenna.
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