Antenna questions

[shawnd]

Yep no documentation on it. I thought this is the stock antenna that Mooney shipped the aircraft with. I am going to go with the CI-122 (https://www.aircraftspruce.com/catalog/avpages/comant122.php) as it makes the most sense to have that range

Btw, great discussion folks! Thank you.

[larryb]

Back to original question #1. You don't want a rod and ball antenna if the antenna is in the slipstream. They have a surprising amount of drag. You want a transponder blade antenna such as the Commant CI-105.  If the antenna is behind the fiberglass belly pan then you can use a rod and ball antenna such as the Ted. I have a  Ted antenna mounted where the marker beacon antenna used to be feeding a GDL-52R. It works very well in that location.

https://www.aircraftspruce.com/catalog/avpages/ted_transponder.php

https://www.aircraftspruce.com/catalog/avpages/comant105.php

[shawnd]

Agree - I do need to install it temporarily to get a test and ferry flight in.

Question: is there a way to externally test, in a price conscious manner, which antenna is actively transmitting? I would like to do this without patching in between the transmitter and the antenna as the COMM antennas on the roof are inaccessible unless I take the interior off.  What I am thinking of is the same type of device that is used to do regular transponder checks but in this case, just simply needs to measure the antenna output.

[EricJ]

18 minutes ago, shawnd said:

Agree - I do need to install it temporarily to get a test and ferry flight in.

Question: is there a way to externally test, in a price conscious manner, which antenna is actively transmitting? I would like to do this without patching in between the transmitter and the antenna as the COMM antennas on the roof are inaccessible unless I take the interior off.  What I am thinking of is the same type of device that is used to do regular transponder checks but in this case, just simply needs to measure the antenna output.

If you have a handheld VHF com radio and a large-ish piece of sheetmetal you can try shadowing the antennas individually with the sheetmetal (hold the sheet close to the antenna with the sheet between the antenna and the handheld), and transmit with the handheld a reasonable distance from the airplane.    See which receiver picks it up more strongly.     Do not do this in a hangar, do it outside where reflections are minimized, preferrably away from metal buildings or in as much clear space as you can manage.    You don't want reflections coming back from behind the antenna.   

[shawnd]

Hmm that’s one way for sure. Problem is I won’t be able test non-COM antennas this way.

[thinwing]

On 5/13/2022 at 7:58 AM, EricJ said:

It's not an impedance matching issue it's just the frequency response of the antenna.    If it is tuned (dimensionally) for 138-174 MHz it'll have less gain at frequencies outside that range as the response tapers off.

35 year career wireless R&D engineer here.   

Why this particular antenna is indicated to be tuned for that range and TSO'ed for VHF aviation comm use is unknown to me.   I suspect it's just that it's a little shorter so that it fits underneath for belly mount.

More pedantic....above chart shows two bands occupied by vhf amatuer radio bands in the 2 meter (144 to 148 mhz)and 1.25 cm (222 to 225 mhz).Also a poor impedance mismatch wont effect recieve decible but on transmit a really poor SWR say 3 to 1 means high percentage of radiated power is reflected back down the coax to the radio.There ,it has no where to go but backup to the antenna,than back again...this resonating wave is what causes finals to burn out.Antenna tuners (matching network impedance )can balance this out...our 10 watt am transcievers are desighed to put up with a lot of mismatch but in a poor match ..10 watt transciever has the same effective range as a low power handheld  ...also a HAM....amatuer extra class

[Pinecone]

On 5/25/2022 at 5:14 PM, thinwing said:

More pedantic....above chart shows two bands occupied by vhf amatuer radio bands in the 2 meter (144 to 148 mhz)and 1.25 cm (222 to 225 mhz).Also a poor impedance mismatch wont effect recieve decible but on transmit a really poor SWR say 3 to 1 means high percentage of radiated power is reflected back down the coax to the radio.There ,it has no where to go but backup to the antenna,than back again...this resonating wave is what causes finals to burn out.Antenna tuners (matching network impedance )can balance this out...our 10 watt am transcievers are desighed to put up with a lot of mismatch but in a poor match ..10 watt transciever has the same effective range as a low power handheld  ...also a HAM....amatuer extra class

Only if the transmitter end does not have a good matching network.  If it does, the power is reflected and added to the power at the antenna and is radiated.  What you lose if the  reflected power going twice the length of the coax, and the coax losses from doing so.

[EricJ]

4 hours ago, Pinecone said:

Only if the transmitter end does not have a good matching network.  If it does, the power is reflected and added to the power at the antenna and is radiated.  What you lose if the  reflected power going twice the length of the coax, and the coax losses from doing so.

It depends on the system.   Many systems have power amplifiers that don't tolerate mismatches and the reflected energy can damage the PA.    Even in many handheld systems there are warnings to not transmit without an antenna (or some reasonably matched load) connected.    It used to be a sure way to destroy a PA.

These days many PAs are protected by design all the way from short (zero impedance) to open (infinite impedance), so they just adapt to whatever is there.   Efficiency will always be best in some matched range, and that is still true for transmission lines and antennas, but not as big of a deal as it used to be.   Matching also minimizes distortion, which is more important with many of the high-order modulations that are used today.

I visited an antenna farm on a mountain peak site yesterday and there was discussion about many systems moving to powered fiber cable running from the shack to the antenna on the tower, eliminating the traditional transmission cable running up the tower and moving the modem from in the shack up to the antenna.   This solves a lot of problams and improves efficiency, but makes the ODU (outdoor unit) (or RRU, remote radio unit, as they call it now) more complex.   My company proposed something similar about ten years ago when power over ethernet became available, so the transmission line could be replaced by a PoE cable, but the available power levels weren't quite there for many applications.   

But our aviation VHF voice analog systems are very, very old school, and I always assume the old rules apply, i.e., don't transmit without a load, match the impedances as closely as possible, etc., etc., as they'll more likely get the best results.   Until we get modern digital modems with radios connected directly to the antennas, the old rules will still apply.   In US general aviation I don't expect many changes in my lifetime.

[Pinecone]

We are saying the same thing. 

I did say WITH A PROPER MATCHING CIRCUIT.

[carusoam]

You guys really know your antennas…

@Gagarin probably would enjoy the discussion as well…. He likes these details as well…. And has been known to install some funky antennas for over water / cross the Atlantic types of flights… 

Best regards,

-a-