Why not one?

After reading a few posts on the dual comm antennas and my limited knowledge of anything in avionics, why couldn't you just use one antenna with two comm radios?

Tbone said:

After reading a few posts on the dual comm antennas and my limited knowledge of anything in avionics, why couldn't you just use one antenna with two comm radios?

Click to expand...

when you transmit, you push a significant amount of power into the antenna. if that were just connected to the other radio, the non-transmitting radio would have it's receiver's front-end damaged. ok, you say, i'll install a switch. problems -- switch would need to be coaxial (for impedance matching and shielding), and if you ever forgot and transmitted on the "wrong" radio (the one without the load), you would most probably damage the transmit section of that radio.

all in all, not a good idea.

One more thought

FWIW That's what I intend to do. One comm antenna on the belly. The downside is, no redundancy, but I'll carry a hand-held for that.

The technical workaround is a duplexer, sometimes called a T/R switch. It's a device that will isolate comm1's receiver from comm 2's transmitter (and vice versa) , and switch the appropriate transmitter to the antenna. The operation is transparent to the operator. If my understanding is correct (an all too often rarity), it doesn't switch the receivers, so you can still receive on both comm's at the same time.

Comant makes one (big bucks): http://www.comant.com/prodpages/ci601.html

And Bob Archer, the wingtip antenna guy, also has one (MUCH cheaper): http://www.aerocraftparts.com/ItemForm.aspx?item=SA-010.

Clear skies,

diplexer

yeah, there is that solution. but for most of us, a second antenna is a whole lot cheaper that a diplexer, provides some redundancy, and fewer points of possible failure. probably costs less than a knot of drag.

I couldn't find any specs on the diplexer from Archer. However, it looks like a passive device. If so, there has to be some insertion loss associated the device. This would degrade performance for both receivers at least some. Both receivers may still work better than one using a mostly horizontal wingtip antenna, though.

I am not sure what the market is for the diplexer. I guess if you only have room for one com antenna or feel that antennas are a real "drag" you use it. I personally don't think the .5 mph or so is worth it. However, I know others have different missions for their aircraft. [ACCENT = "fake french"] VIVA LA DIFFERENCE! [ACCENT = "southern drawl"]

Duplexers only work if receive only

You can only use a duplexer if the two radios sharing the antenna are receive-only, as in the case of a NAV or Glideslope receiver. If you attempt to use the same antenna for two COM radios, then tune them to the same freq, when you transmit on one, you'll blow out the other one. In fact, even with two antennas, they should be at least 3 feet apart,

The only alternative would be to have an RF-frequency switch, so that you could physically switch the antenna between to the two radios. What a hassle!

If you want two COM's, you need to COM antennas. If the 2nd one is not critical, and you don't want to give up the drag, then hide it in a gear fairing. You can make a hidden COM antenna by stripping off 21 inches of shielding from a piece of coax.

Jon,

I'll point you to the urls again:

http://www.comant.com/prodpages/ci601.html

http://www.aerocraftparts.com/ItemForm.aspx?item=SA-010.

Comms (transmit and receive) is what they are made for. They are sometimes called T/R switches (T/R for transmit/receive)


Tony,
You're correct. I'm not doing it to save .5 knots. Nor do I care about the .5 db loss (or whatever). I'm doing it so I don't have to have another ugly antenna and don't have to run another cable. You pay your money and take your choice.

John,
You're correct too that a 2nd antenna is a whole lot cheaper. But IF you wanted to use a single antenna, this is the way to do it.

Regards,
Best,

Has anyone tried this?

jonbakerok said:

You can make a hidden COM antenna by stripping off 21 inches of shielding from a piece of coax.

Click to expand...

Jon, does this need to be grounded to the airframe or just go straight to the radio?

I would be cheap enough to play around with.

Kent

johnp said:

when you transmit, you push a significant amount of power into the antenna. if that were just connected to the other radio, the non-transmitting radio would have it's receiver's front-end damaged. ok, you say, i'll install a switch. problems -- switch would need to be coaxial (for impedance matching and shielding), and if you ever forgot and transmitted on the "wrong" radio (the one without the load), you would most probably damage the transmit section of that radio.

all in all, not a good idea.

Click to expand...

Yes, it's bad (not to mention expensive and smelly) when the Magic Smoke leaks out

Some radios have protection against transmitting without the antenna connected (VSWR protection). This would happen if you had a switch where one antenna was manually switched between two radios.

If you transmitted one one radio while both were connected to the same antenna, ... well, that would be Bad. You'd likely smoke the front end of the second radio.

TODR

Receiver splitter losses...

Typical two-way coaxial splitters have around 3.5 dB power loss to the receivers. That is not good. Go with two antennas for lower loss. If you are not flying an RV4 or RV8, put one on the top aft deck and the other underneath the seats. That way you get maximum RF isolation between the antennas. As you can see, I chose to put my one COMM antenna on the aft upper fuselage skin behind the slider canopy. If I decide to put in a second one, it will be bent whip on the underside of the seats.


As for trying to hide the home-made coaxial antenna next to the gear leg inside the gear leg fairing, it won't radiate well at all. You need a stiff whip antenna out in the clear and away from the gear legs to get a good radiation pattern on transmit and receive.

After 36+ years in the cable, satellite uplink/downlink, and fiber optic communications business, I picked up a few things along the way.

Thanks

for the info. I do believe that I will install two under the fuse. Does it make a difference if they are straight or bent? How much ground clearance is needed or
does it make a difference on a -7?

You really don't want a transmit antenna (less critical for receive, but still applicable) near any metal that is in the same plane as the antenna. Ideal location is perpendicular to any large flat piece of metal which can serve as a ground plane.