Radio antenna separation???

[Hawkeyenfo]

Hi all,

The time has come and 39A is at the avionics shop getting it's "new" radios installed. Shop says that they'll have to relocate both existing antennas They told me the antennas must be 36" apart. Mine are 18" apart now and located on both sides of the fuel tank vent. Is the 36" based on wavelength and if so is that approx 1 entire wave or half? It seems that 18" may have been picked for a reason....half a wavelength? I'm installing a King 135 and 197.

Any ideas???

Thanks!! Chris

[hilltop170]

Chris-
I'm no radio expert and consider them to be one of the great mysteries in life. But from my old C.B. days, I did learn a little about antennas trying to get the most performance out of 5 watts on cars, houses, and airplanes.

I can't see where the separation would be that critical unless you transmit on both radios at the same time which you're not going to do. In the old days when radios had wide frequency tolerances, bleed-over was common but no longer.

The separation on my com antennas is 24" and has been that way since 1979 when a KX-170B and a Cessna 300 Nav/Com were installed. The 300 Nav/Com was crystal and not sure about the KX-170B but it sure wasn't latest technology. I never experienced any problems due to antenna spacing. With your radios, I bet there would not be a problem either.

If it was my plane, I would run new coax and make sure the antenna grounds are good, then try the antennas just the way they are. If there is a problem you can always change them later. I know I wouldn't want to punch any more holes than necessary in my plane, especially in the roof. Good luck!

[GAHorn]

VHF comm antennas must be mounted at least 12" from transponder/DME antennas (pulse equipment), otherwise they are fine as long as they are 1/4 wavelength from each other. (i.e., approx. the same as their total length. so if your anntenna is 20" long, then mounting it no less than that distance from another similar antenna should pose no problem. Placing comm antennas too close together can cause switching diode failures.)

Traditionally, comm antennas were placed no closer than 3 feet from each other. This was critical during the days of vacuum tube avionics, but modern digital avionics have much better rejection of spurious signals and operate at lower transmit wattages, and unless you've experienced previous problems 18" between comm antennas should be fine. (The old 36" rule is a left-over from dinosaur-avionics days.)

(Distance from other speical purpose antennas may obviate this info. For example, DF antennas require greater spacing from comm antennas due to a blanking effect. Ordinarily, Glide Slope, Loran, ADF, and Stormscope antennas are unaffected.)

[hilltop170]

What he said.

[n2582d]

If is only a requirement from vacuum tube day it's interesting that Comant says the following in their FAQ:

How much distance should there be between antennas ?
You should maintain 36 inches as a minimum distance between antennas. Some antennas, such as SATCOM, may require more. Refer to each manufacturer's installation guidelines.

[GAHorn]

If is only a requirement from vacuum tube day it's interesting that Comant says the following in their FAQ:

How much distance should there be between antennas ?
You should maintain 36 inches as a minimum distance between antennas. Some antennas, such as SATCOM, may require more. Refer to each manufacturer's installation guidelines.

Comant has no control over what year-model or type radio their antennas are connected to. They are relying upon "conventional wisdom" ...which works fine, even today. Their advice to refer to each mfr's guidelines is good.
But it is not an absolute that comm antennas be spaced 36" apart. It was "good practice." If a person has had antennas installed on his plane for years and years that are only 18" apart, and he wishes to replace those antennas with others of similar style and service, there is no reason to have to drill new holes (and close off the old) simply to space the new antennas 3 feet apart.

[hilltop170]

A strange thing happened after the WAAS upgrade was installed on my Garmin GPS. When talking on the radio, I noticed the GPS navigation function would fail any time either com radio was keyed on any frequency from 120.6 to 121.7. I had never seen that before on any GPS and it would not be good if you're shooting an approach at the time.

Today the radio shop came over and I demonstrated it to them. They called Garmin Tech Service who said the WAAS upgrade increased the GPS sensitivity from 17 to 26db (? I'm not sure of the units) which makes it more susceptable to interference.

They said the problem could be caused by the ELT re-radiating a signal at freqs close to 121.5 due to the location of the ELT antenna being too close to the GPS antenna. They are about 24" apart.

Garmin suggested putting a notch filter on the GPS coax on the back of the GPS which was tried first and did not correct the problem. Garmin then said put another one on the ELT coax connection at the ELT. It did the trick and no more dropped GPS signals when transmitting on the radios.

The radio shop had the notch filters in stock. It is a small 'tee' shaped filter with BNC connectors.

Another weird solution for an odd problem to keep in your bag of tricks should that ever happen to you.

[jrenwick]

I had a very similar problem after installation of my GNS430W last June. Garmin expects this to happen with a certain number of VHF frequencies, and it's a normal part of the installation to check for it and install a notch filter on the antenna coax. My shop did the check but didn't see the problem somehow. I told them about it, they installed the filter in about 5 minutes, and the problem was solved.

[cessna170bdriver]

Another weird solution for an odd problem to keep in your bag of tricks should that ever happen to you.

When it comes to interactions between various pieces of RF equipment, I've come to expect "wierd" as "normal". When my King KN64 DME is tuned to certain channels, my Narco 150TSO transponder replies to its interrogations. I know that DME and Transponder frequencies are close to each other, but I've never been able to establish a pattern as to which channels cause it and which ones don't. In any case it doesn't seem to cause any operational problems. (Yes, I know DME is only a couple of generations past airway light beacons, but until Santa drops me a 5-digit stocking gift, I'm gonna hang on to it.)

Miles

[GAHorn]

Another weird solution for an odd problem to keep in your bag of tricks should that ever happen to you.

When it comes to interactions between various pieces of RF equipment, I've come to expect "wierd" as "normal". When my King KN64 DME is tuned to certain channels, my Narco 150TSO transponder replies to its interrogations. I know that DME and Transponder frequencies are close to each other, but I've never been able to establish a pattern as to which channels cause it and which ones don't. In any case it doesn't seem to cause any operational problems. (Yes, I know DME is only a couple of generations past airway light beacons, but until Santa drops me a 5-digit stocking gift, I'm gonna hang on to it.)

Miles

Miles, interference between DME and TXDR is common, expecially among older equipment. There is a "suppression" channel that interconnects your DME/TXDR that can be enabled. It's a simple coax connection between the two radios.
It was standard practice until just recently for the channel to be enabled, but sometimes it's overlooked, expecially when the two pieces of equipment are installed at different times/by different shops.

[johneeb]

Another weird solution for an odd problem to keep in your bag of tricks should that ever happen to you.

When it comes to interactions between various pieces of RF equipment, I've come to expect "wierd" as "normal". When my King KN64 DME is tuned to certain channels, my Narco 150TSO transponder replies to its interrogations. I know that DME and Transponder frequencies are close to each other, but I've never been able to establish a pattern as to which channels cause it and which ones don't. In any case it doesn't seem to cause any operational problems. (Yes, I know DME is only a couple of generations past airway light beacons, but until Santa drops me a 5-digit stocking gift, I'm gonna hang on to it.)

Miles

Miles,
Isn't it interesting how fast we become blase about the information that is available to us today. Until the advent of handheld GPS I used to be envious of anyone who had a DME. Shortly a generation will look on our generation with awe at having been able to work with DME, similar to the way we look at Bllueldr's generation for their skill with a radio range.

[GAHorn]

... Shortly a generation will look on our generation with awe at having been able to work with DME, similar to the way we look at Bllueldr's generation for their skill with a radio range.

Don't forget about Montgolfier and my skill with hot air!

[blueldr]

Time flies!
During WWII the military came out with VHF comm radios. They had 4 (four) channels. Post WWII they came out with a comm radio with 8 (eight) channels. The common comment on the flight line was "What in hell are they going to do with 8 channels?"

After all, the civil aircraft all operated on HF and the standard frequency for an aircraft transmitter was 3105KC. Everyone was on that frequency. You listened on a tuneable receiver on frequencies from 200 to 500 KCs. A common tower frequency was 207.

When civil VHF became available, they had 90 channels. "For christs sake how in hell will we ever be able to keep tract of what we're doing on the radio?". In a few years they were at 360 channels and it wasn't long before 760 became available.

In Europe they now have 8 1/3 spacing which gives them about 2280 channels. No doubt we will be into it soon.

And the part that really torques my butt is that it seems like everyone in the Sacramento Valley is stuck with 122,8

[cessna170bdriver]

Miles, interference between DME and TXDR is common, expecially among older equipment. There is a "suppression" channel that interconnects your DME/TXDR that can be enabled. It's a simple coax connection between the two radios.
It was standard practice until just recently for the channel to be enabled, but sometimes it's overlooked, expecially when the two pieces of equipment are installed at different times/by different shops.

I made that connection and it didn't work, although as I remember I made it with twisted pair, not coax. Not sure if the problem is on the DME or transponder end. ATC has no problem with the transponder, and I have no problem with the DME, so I just left it.

Miles,
Isn't it interesting how fast we become blase about the information that is available to us today. Until the advent of handheld GPS I used to be envious of anyone who had a DME. Shortly a generation will look on our generation with awe at having been able to work with DME, similar to the way we look at Bllueldr's generation for their skill with a radio range.

A friend of mine recently retired as an instructor at the Air Force Test Pilot School, and now serves as "Chief Instructor" at the Edwards Aero Club. In the last month or so they took delivery of a new 172 with a G-1000 (the ONLY way you can get them these days). He says the SOP suggested by Cessna is to engage the autopilot above 500ft. If this keeps going, the new generations will be in awe that we once were able to manipulate aircraft controls. (BTW, I'd love to find an addon to to Flight Simulator so I could give the AN Range a try. I have a book THROUGH THE OVERCAST, by Assen Jordanoff published in the 1930s that details the procedures. )

After all, the civil aircraft all operated on HF and the standard frequency for an aircraft transmitter was 3105KC. Everyone was on that frequency. You listened on a tuneable receiver on frequencies from 200 to 500 KCs. A common tower frequency was 207.

I'm not sure if it was the HF or early VHF sets, but Dad talks about an old radio where you had to physically change crystals on the front of the radio to change channels. If a flight required multiple channels to be used, you kept the crystals on a string tied to the radio to keep track of them.


Miles

[GAHorn]

In the lobby/atrium of my workplace (SimuFlite, DFW) there is a 1942 Singer Link trainer on display. Next to it, framed on the wall, is a Sept. 1940 Dallas-Ft.Worth Sectional Chart that I have on loan to the company.

It has HF freq's for the control twr at Love Field, and colored airways with AN-ranges, and Dept. of Commerce AirWay Flashing Beacons (which had Morse-code numeric-systems) for the mail-routes.

None of the present man-made lakes. Nothing but a vast expanse of farmland between Dallas and Ft. Worth. Price of the chart: 25-cents.

(I also have a copy of the original AN approach into Dallas Love Field. No timing remarks were suggested, only distances.... only BluElder would know how they determined that in the days of sun-dials, before bearing-pointers.)

More seriously, ... a good pilot could determine his position relative to any AN range station by making only 4 timed-turns. These days, I have trouble with clients who have absolutely no idea what to do if the FMS fails on takeoff.