Belly-mounted comm antenna?

[schomburg]

We are considering the addition of an "extra" comm antenna in a 182P, cabled only to a BNC coaxial connector on the instrument panel, to get improved comm performance from a backup handheld radio. There is a choice of rooftop or belly mounting. The belly-mount option seems to have these advantages:

[1] Possibly simpler installation, since headliner removal and windshield-post 'cable-snaking' are not involved.

[2] The antenna-pattern main lobes would ne 'out-and-down', instead of 'out-and-up' as for a rooftop location, which may be a benefit with a low-transmit-power handheld.

[3] Ice accumulation is less likely, due to engine cooling airflow. (Not that we should be there anyway!)

A belly antenna will obviously gather some oil and grime, and it will need careful placement to avoid runway-scraping.

Any other thoughts on this?

Dick Schomburg
Hillsboro, Oregon

 

[Eamon]

The belly is the best place for an ant, so go for it. They do make special com ant for bellys. You might want to hook it to comm 1 & use one of the top ant for the handheld.

I never knew why they would put an ant on top when you are trying to transmit to the ground anyway.

Make sure you put a doubler in & you might need a 337 & your FSDO might require you to prove that the area is strong enough so...

It is best to install it in an old ADF hole or the position where Cessna would have put another ant, Or the mirror side to the Transponder ant, etc. That way you can be sure it will still be strong.

Good luck

Eamon

 

[schomburg]

Eamon,

Thanks! I think one advantage of a rooftop mount comes when you need to contact FSS or Clearance Delivery folks while still on the ground, as from a non-towered airport. Unless there is an RCO nearby, the height is a plus. For that reason I'd not put Comm1 on the belly; in our case at UAO it's already somewhat iffy getting thru to clearance delivery.

Actually, the best setup would put the antenna-pattern main lobe on the horizon, since that helps most to reach a distant station. But that takes a longer whip, (the so-called 5/8 wavelength design), and anyway a wing is only an approximation to the "infinite ground plane" on which these hypotherical best cases are based.

The a/c is currently in Visalia for an engine overhaul and annual; we're thinking of having the antenna work done there before we pick it up.

Dick

 

[wsuffa]

I have belly mounted aux comm antenna that runs to a BNC jack on the panel. It works well. Until I got XM, I used a small multi-band receiver to listen to FM broadcasts. When I go into SDL, they want you to get clearance before engine start - with a handheld and that bottom-mount antenna - no problem and no battery drain.

I say go for it.

 

[Richard]

Wait a minute. What's going on here? When I posted my radio comm problems on AOPA recently the belly mounted comm ant was identified as a possible suspect. Now y'all are saying it aint so?

In fact, on the advice of several knowledeagable folks I am seriously considering moving the ant to the top, aft of the cockpit.

 

[schomburg]

Richard,

Before making that move, it might pay to inspect the existing antenna cable very closely. I missed that AOPA thread, but cable and/or connector issues can cause comm problems that may not involve the antenna itself, or its location.

Look for insulation dents, sharp bends, or excessively-tight clamps, for example, any of which, over time, can lead to plastic flow.

As an intermediate step, you might replace the entire cable assembly before moving the antenna. And if you do move it, use brand new cable and connectors!

Dick Schomburg
Hillsboro, Oregon

 

[wsuffa]

Wait a minute. What's going on here? When I posted my radio comm problems on AOPA recently the belly mounted comm ant was identified as a possible suspect. Now y'all are saying it aint so?

In fact, on the advice of several knowledeagable folks I am seriously considering moving the ant to the top, aft of the cockpit.

Richard, my main antennas are on top. There really was no room for the backup up there. I don't think I'd want my main antennas on the bottom unless there were no choice.

 

[Richard]

Richard, my main antennas are on top. There really was no room for the backup up there. I don't think I'd want my main antennas on the bottom unless there were no choice.

Bill, but if it's a bad location to start with... Or is it a bad location? I'm just trying to get to the bottom of it. I've heard a lot of various advice.

 

[wsuffa]

Bill, but if it's a bad location to start with... Or is it a bad location? I'm just trying to get to the bottom of it. I've heard a lot of various advice.

Richard, bottom line (no pun intended) is that an outside antenna *anywhere* on your aircraft will be many, many times better than the rubber antenna in the cockpit.

There really are a lot of factors that go into it - my bottom mounted antenna works fine for the handheld. I have a retract gear plane, so in flight, there's nothing but a good ground plane down there.

What's most important is getting a good antenna, with a good ground around it, and a good SWR back at the antenna jack. IIRC, you main goal is to use this with a handheld, and the most critical time in flight for you is within 10 miles or so of the airport. A good, properly matched antenna should work fine on the bottom of the plane. OTOH, if you're going for maximum range, and it's a metal plane, I'd probably opt for putting it on top. I do a LOT of long cross-country flying, I'm often pushing for maximum radio range (and sometimes that's not good enough).

Having said that, I've run into situations where the top-mounted antennas can have some reception issues talking to ATC in certain areas. I've chalked that up to their postion near the vertical stabilizer.

There are a lot of factors that affect antenna placement, which is why you're getting a lot of different answers. My personal preference is to put the Com antennas on top of a metal plane, one antenna per radio, with the cable run to the radios.

Having said that, I would much rather have a bottom mounted antenna on a metal plane run to the handheld antenna jack than to put a splitter or switch from one of the top antennas (there is enough loss in the switch to nullify the benefit of the top mount). On a fabric or composite airplane, you want the antenna placed wherever you can get the best groundplane (and at VHF frequencies, the ideal groundplane is at least a couple of feet of metal, screening, or foil). Ground plane requirements get smaller as the frequency gets higher, so DME and Transponder antennas have smaller ground planes.

Bottom line: there is no "one size fits all" answer. It's all in the mission requirements - sorta like buying a plane - and everyone will have an opinion.

bill

 

[Richard]

Bill, thank you for your obviously well thought out response. FYI, I have a fabric covered plane and it's the primary comm I'm concerned about. After thinking about it, the reason the ant is on the belly (bottom of cowl) is because it's the only metal large enough to act as a ground plane. Your latest response helped me figure that out.

There is a likely place above the aft cockpit (exterior) for ant but would involve running wire (think zip strip for signal strength) along the fuselage and outward from ant for adequate ground plane. But that would be if running all new wires and connectors for comm and intercom doesn't work.

 

[Stache]

Installing a com ant. will require a field approval on a FAA form 337. I have field approved several of there over the years (No big deal). The 337 will required you to fill out all 16 steps for the Instruction for Contuined Airworthiness (ICA) on the back in block 8. The ICA is one thing the FAA got right as all 337 are the same in format nation wide.

When you install the ant. you will be required to install a backing plate. If you follow the guidance in AC 43.13-2A chapter 3 you will not have any problems. Next take your aircraft to a avionic shop to check the coax cable before install it. The length and location will effect the coax cable.

Stache

 

[wsuffa]

Bill, thank you for your obviously well thought out response. FYI, I have a fabric covered plane and it's the primary comm I'm concerned about. After thinking about it, the reason the ant is on the belly (bottom of cowl) is because it's the only metal large enough to act as a ground plane. Your latest response helped me figure that out.

There is a likely place above the aft cockpit (exterior) for ant but would involve running wire (think zip strip for signal strength) along the fuselage and outward from ant for adequate ground plane. But that would be if running all new wires and connectors for comm and intercom doesn't work.

Richard, how good is the ground and groundplane? If' you're having reception problems with the existing antenna, that and the wiring (which you're already checking) would be the first place I'd start.

 

[Richard]

Richard, how good is the ground and groundplane? If' you're having reception problems with the existing antenna, that and the wiring (which you're already checking) would be the first place I'd start.

Bill, I don't know. What is the test to check for ground plane called and what equipment is required? It's probably been done already, you'll just have to refresh my memory what test it is.