Marker Beacon Antenna Placement

MBDiagMan

MBDiagMan

Final Approach
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I have procured an early tech whip marker beacon antenna out from under an old Cessna 310. From what I have read, recommendations are to mount all antennas at least two feet from other antennas.

My mechanic, who admittedly has only a fundamental avionics knowledge wants to mount it off to the side of the ELT antenna. Both of them will be on the underbelly. He is going to help me rivet a doubler where I will mount it, so he is concerned about access for driving the rivets. He is saying that the frequencies are different enough that it won't be a problem and he might be right. The MB is 75 MHz while the ELT is 121.5 and 243 MHz, not harmonic with 75.

Any experience with this?

Thanks for any comments and sharing of experience.
 
He's an idiot. The relative frequencies aren't the issue. However the tuning is probably not sufficiently critical on either one such that close proximity of the other is going to cause a problem. You also don't have to worry about one transmitting too close to the other as if your ELT is transmitting, the MB receiver is probably the least of your worries.

My big question is why on earthy you would put the ELT antenna on the bottom of the aircraft where it is most likely to be damaged in a crash when you need it the most?
 
flyingron said:
He's an idiot. The relative frequencies aren't the issue. However the tuning is probably not sufficiently critical on either one such that close proximity of the other is going to cause a problem. You also don't have to worry about one transmitting too close to the other as if your ELT is transmitting, the MB receiver is probably the least of your worries.

My big question is why on earthy you would put the ELT antenna on the bottom of the aircraft where it is most likely to be damaged in a crash when you need it the most?
Click to expand...

My question is why install a marker beacon at all? They are going away at a rapid pace.
 
Thanks for the response Ron!

No he's certainly not an idiot and as I said he readily admits his lack of avionics knowledge. I think I am probably the idiot because your post made me realize that the antenna underneath is probably the transponder and the one on top is the ELT. Since the transponder is frequently transmitting, having this antenna close might not be a good idea.
 
The marker beacon is already installed, but the signal was split off the NAV/GS antenna so it's too weak. I am installing a dedicated MB antenna for my instrument training. There are several ILS approaches in the area with MB transmitters still active.
 
It's easy to differentiate a DME or Transponder antenna from an ELT. The former are tiny (either a very short, like an inch, whip or a plastic blade no more than a few inclhes). Elts are typically the same as com antennas (although typically of more cheesy construction) bout a foot and a half whip.

I'd be very concerned putting anything metal (antenna or other protruberance) in very close proximity of the UHF transponder or DME antenna.

I'm confused about the "weakness" of your MB signal. It only needs to fire off when directly overhead. There aren't too many fan markers still kicking around (if any) that need more sensitivity.
 
You are right about the Xponder and ELT antennas.

I theorize that the weakness is related to a the splitter that was trying to split a marker beacon signal, a glide slope signal and a VOR signal, ALL off of a dipole antenna mounted on the top of the vertical stabilizer.

I have connected a splitter for the GS and VOR's, now it's time to connect the separate MB antenna.
 
No worries about the xponder causing RMI with the MB, as the MB is khz, and the xponder is ghz. they are way from being resonant.
 
bluerooster said:
No worries about the xponder causing RMI with the MB, as the MB is khz, and the xponder is ghz. they are way from being resonant.
Click to expand...

Resonance AGAIN is not the only issue. Further, the difference in frequencies is NOT going to be the end-all in determining if this thing is going to be resonant (or close enough) at the frequencies involved. While most marker beacon antennas even the whips are not strict quarterwaves. Dropping 200W even at a partial match into them isn't going to be nice. And as stated, putting a large hunk of metal next to a UHF antenna isn't a good idea no matter how it's tuned.
 
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Thanks again for more responses!

As I am understanding it, the MB is a 75 MHz carrier frequency. I plan on moving it away from the Xponder antenna. I am anxious to get to the airport tomorrow and take a fresh look.
 
bluerooster said:
No worries about the xponder causing RMI with the MB, as the MB is khz, and the xponder is ghz. they are way from being resonant.
Click to expand...

It always amazes me the lack of knowledge on newsgroups.

Marker Beacons are 75 Mhz.. Transponders are 1030/1090 Mhz. Or, if you prefer, 0.075 GHz and 1.03/1.09 GHz.

Resonance isn't the problem. Large metal masses (a 39" Marker Beacon antenna) in proximity to a 3" transponder antenna is. As is a 200 watt peak pulse transmitter next to an antenna that needs microvolts to activate it.

Jim


.
 
weirdjim said:
It always amazes me the lack of knowledge on newsgroups.

Marker Beacons are 75 Mhz.. Transponders are 1030/1090 Mhz. Or, if you prefer, 0.075 GHz and 1.03/1.09 GHz.

Resonance isn't the problem. Large metal masses (a 39" Marker Beacon antenna) in proximity to a 3" transponder antenna is. As is a 200 watt peak pulse transmitter next to an antenna that needs microvolts to activate it.

Jim


.
Click to expand...



Heh. To heck with newsgroups, how about the certified mechanic who just recommended he do it? LOL!
 
Well Jim just repeated what I said. Jim might know a few things about radios. He spent years running an avionics company in addition to being quite involved in a number of other radio engineering projects.
 
MBDiagMan said:
The marker beacon is already installed, but the signal was split off the NAV/GS antenna so it's too weak. I am installing a dedicated MB antenna for my instrument training. There are several ILS approaches in the area with MB transmitters still active.
Click to expand...

For what it's worth, I just went through instrument training and had my checkride about 6 weeks ago, and other than knowing what a marker beacon was (some old thing like an NDB!), there was no other use for it. The examiner asked me what the term "LOM" on a plate meant, and that was the extent of it on my checkride.

Don't throw good money at bad...

[EDIT] Is it coincidence that your username is MBDiagMan?!?!

-Andrew
 
ahkahn said:
For what it's worth, I just went through instrument training and had my checkride about 6 weeks ago, and other than knowing what a marker beacon was (some old thing like an NDB!), there was no other use for it. The examiner asked me what the term "LOM" on a plate meant, and that was the extent of it on my checkride.

Don't throw good money at bad...

[EDIT] Is it coincidence that your username is MBDiagMan?!?!

-Andrew
Click to expand...

:rofl: The MB in my moniker stands for something besides Marker Beacon.

I am not putting a significant amount of money into making the Marker Beacon work. If I can make it work, I will be able to do an ILS approach at a nearby airport for training and on the checkride.
 
weirdjim said:
It always amazes me the lack of knowledge on newsgroups.

Marker Beacons are 75 Mhz.. Transponders are 1030/1090 Mhz. Or, if you prefer, 0.075 GHz and 1.03/1.09 GHz.

Resonance isn't the problem. Large metal masses (a 39" Marker Beacon antenna) in proximity to a 3" transponder antenna is. As is a 200 watt peak pulse transmitter next to an antenna that needs microvolts to activate it.

Jim


.
Click to expand...

It didn't dawn on me the output of the xponder being 200w peak.
But I also suspect that the mechanic that suggested installing the MB antenna on the belly, wasn't intending to install it right next to the transponder antenna. But has been interpreted that way.
Most aircraft that I've seen (and all that I've owned) have the MB and the Xponder antennae on the belly of the aircraft. Albeit not right next to each other.
 
Okay, my mechanic friend helped me put a doubler on the belly behind the battery box which is just behind the cargo hold behind the seats. This is at least four feet, but probably more like six feet from the Xponder antenna and at least three feet from any other antenna on the aircraft.. We got done about 12:30 today before going to lunch. After lunch I got the coax routed and most of the interior back in that had to come out.

I hope to finish fitting and terminating the antenna tomorrow in time to fly over and test everything if the weather holds out.

Thanks again for all the responses.
 
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