The International Cessna^® 170 Association

Antennas?

I'm having, or rather some approach and tower controllers are having difficulty with my radio transmission and reception.
I can talk to aircraft that are 30-40 miles away and some ground facilities that are 30-40 miles away. It's when I get close to a ground station that difficulties arise. I think it's becaue my stainless steel rod antenna is mounted at the rear of the cabin... basically even with the trailing edge of the flaps.
Line-of-sight to a ground station is being blocked by the wings and fuselage.

My idea is to move the antenna forward to a spot about 6" behind the windscreen.

Thoughts?

My COMM is further aft. About mid point from the trailing edge of the flap and the beginning of the rudder.

I have the same problems when trying to call Patomac (DC SFRA) from the West, they can't hear me until I turn North or South but I can hear them fine.

My opinion is the same as yours. My antenna transmit is being blanked out due to its location.

Luckily, my GPS puck us up on the cabin, just behind the fuel vent and the bolt pattern is exactly the same

Thinking of replacing the left one with a newer antenna w/a BNC connector.

Well I'm certainly no expert but I have a friend that is and this is what he had to say.

Your antenna on the top of the plane is being blocked from the ground by the plane to stations close on the ground. Reception will be OK to anything at the same altitude or above or farther away on the ground. In effect the antenna is aimed at the sky not the ground. Moving the antenna to the bottom of the plane will solve this problem for communicating to anything underneath the planes altitude.

This is why airplanes with two comms generally have one antenna on the top and one on the bottom so you can use the one that has the best reception for the position you are in relative to the receiving antenna.

His recommendation if you only have one antenna is to put it on the bottom of the plane. With it on the bottom your worst transmission/reception will be to tower while on the ground at the airport or aircraft well above your altitude. Most of the time we want to talk to stations under us however.

As for moving the antenna to the front of the wing he feels will at best will be a minimal improvement if any improvement at all and then only to stations in front of the plane.

The BNC shouldn't make a difference unless of course you want to unplug and plug in the antenna all the time.

Just today I transmitted and recieved over 100 miles between both my Cub and my 170 and they both have a stanless whip like yours. The key is not to have a modern fiberclass shell with a wire inside or a BNC connector but to have either antenna installed correctly. If the connection of either type are not good or the ground plane is bad, new or old won't work.

In your case are the connections i.e. ring terminals good and clean? How about the coax shield to ground plane i.e. the aircraft fuselage?

The "BNC" connector comment is telling.... The plain steel "rod" (radiator) antenna is usually a 1/4-wave antenna and has no "gain", while most "BNC" equipped antennas (which are usually metal radiators encased in foam and reinforced with fiberglass) have Gain designed into them. This results from "gain" in capacitance and diminished electrical-loss-of-signal which occurs to plain-rod/radiator antennas. (The plain antenna "leaks" signal back into it's own mount, while the fiberglass antenna is insulated from that problem.)
In other words, an antenna with "gain" is more efficient than plain, monopole antennas. (Hey! I used to be a C-B nut, and if you want to know about antenna gain/loss/SWR (standing-wave-ratios) then you need to find a CB radio nut!)

"Gain" is a term which describes an increase in performance over plain-radiator antennas. An antenna with gain (usually measured in decibles or "dBs", will broadcast and receive farther/thru more interference than a non-gain antenna, other conditions being equal.

The other situation, one to which Bruce's friend was referring, is the radiation pattern of the antenna. This is affected by what is known as a "ground-plane". Think of it this way: If you skip a rock off the pond, the rock tries to bounce back up into the air and goes farther. Your wing is like the pond, and the radio broadcast signal is the rock. If the antenna is mounted on top of your wing, it will broadcast farther above your wing than below it. If the antenna is mounted beneath your airplane, it will broadcast better to receivers below the airplane. (There's a reason transponder antennas are mounted on the bottom.) This is also a form of "gain".
Small airplanes like ours, which spend the predominant part of their lives below 12k feet are not likely to benefit from bottom-mounted antennas as much as King Airs/Jets which run around in the Flight Levels. Our Cessnas are also more likely to be operated where grass/weeds/rocks and armadillos might strike and injure a bottom-mount antenna, not to mention the loss of signal from all the engine oil and exhaust residue which may build up on it. When you're within 5 miles of the tower (such as when taxying around on the ground) it rarely makes any difference where your antenna is mounted. It probably makes more difference if you're trying to talk to them while you're parked in a "dead spot" behind the mx hangar...move over 100 feet and try calling them again before you start relocating antennas on the airplane.

Another matter which affects broadcast capability is your transmitters power output. This is typically measured/described as power in WATTS, and usually measured at the point of transmission cable/antenna cable outlet of the radio, and is called PEP for (Peak Envelope Power). This is usually limited by design so radios don't interfere with other nearby radios.

Check YOUR transmitter's output (see the specification brochure which came with it.) Typical low-cost aircraft radios are rated at 5 watts PEP, while better units are rated at 10 watts PEP, and higher quality units (designed for high performance, high altitude aircraft) may be rated at 20 watts or higher.

The FIRST thing you should do is check your antenna-ground to make certain it is properly grounded to clean connection to metal on the airframe. If not, then it will have poor "ground plane" and will not broadcast very far. Also, a related problem will be the cable and connectors between your radio and your antenna. Old cable, and multiple connectors will steal the power from your transmitter. GET RID of unnecessary connectors. Also, do not fall for the old trap of having a "splitter" or second radio and/or antenna attached to the same radio. (Some folks have added a connector so they can patch-in their handheld radio to their outside antenna which is already in-use by their main radio. This deteriorates the performance of their main radio output, and also hazards the final amplifier of BOTH radios.
The NEXT thing to do is have your transmitter checked for power output to determine if it's final amplifier is putting out rated power.
The THIRD thing is to use a field-strength meter (cheap from Radio Shack or borrowed from any CB-Nut) and walk around your airplane while your transmitter is activated to determine the pattern of broadcast. Only then will you know if antenna relocation will improve anything for you. (I suspect your location is just fine...your problem is poor transmitter output, poor antenna performance, or leaking antenna cable.**)

**While on the subject of antenna cables... there is a recent improvement in design, and if you find your cable is OLD...and if you replace it, put the good stuff in there. The OLD cable nomenclature was RG-58 while a more modern/more efficient cable these days is RG-400....it has less loss of signal. Also, if you have multiple BNC connectors between your transmitter and your antenna, get rid of them. Each connector results in loss of signal...eventually you may only put out a fraction of your transmitter's power...AND... transmission losses, such as reflected power and connector losses, is hard on your transmitter and will eventually result in early failure of the radio.

Also, occasionally it is found that a radio transmitter is connected to an improper-length antenna. While an old LORAN antenna may LOOK like a COMM antenna.... if it is so used, it will result in huge loss of signal and MAY damage the transmitter due to power being reflected back into the unit. This will overheat the final amplifier and eventually cause failure.

Anyway.... I didn't mean to tell you how to build a clock when all you want is to know the time...but relocating your antenna a few inches or a foot will unlikely solve the probem any more than cleaning up the electrical-contact with it's present location. (It may mislead you into believing the location solved the problem when in reality what you did was give it a fresh, un-corroded mounting which improved it's ground-plane/radiation-pattern. Address all the issues I previously mentioned, first.)

Here's where my antenna is mounted (right next to all the water-spots that dried before I wiped it down). It's a Dorne and Margolin C63-1/A for a top-mount, ...they also make a C63-2/A for a bottom mount. It actually has no more gain than a standard, steel, qtr-wave antenna, and it's driven by a 10-watt Narco 810+, but I've NEVER had any problems from any facility whether I'm airborne or not, regardless of distance or weather, and it's mounted about the same place as yours.



Here's the technicals and pricing (about $150) from Spruce:
http://www.aircraftspruce.com/catalog/a ... mC63_1.php

Here's the bottom mount version (about $200):

I have never had any problems communicating in the air or on the ground in the 39 years (on and off) I've flown my 170. During that time there has been a Cessna Nav-Com 300, a KX-170B, an Icom A200, and a Garmin 430 installed as well as an Icom handheld using the external antenna. The antennas have always been in the same spot on top of the cabin just behind the wing spar. If you are thinking of changing antenna location, you might consider that spot.

If there are problems with reception, like George said, I would suggest it is not the location of the antenna but condition of the radio, coax, bad antenna ground, or some other system problem that I would check-out first.

I personally would not put a com antenna on the belly.

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Aryana wrote:Here is mine.

Have you ever had any trouble with them in that position?

Is that your ELT antenna half-way between the com antennas?

I would not change any antenna position at all. George is correct. I would first pull the ground on the antenna and look for corrosion. That is the biggest problem with comm antennas. You will save a lot of money if you start looking for this first. JMHO. Then I would look to replace the coax cabling and the connectors. That is the most common problems with antenna reception. The question to ask is, Has it always had bad reception even before you owned it, chances are it did not. Good luck, Doug

Aryana wrote:Here is mine.

That's the same position as mine and in 17 years never a problem transmitting or receiving.

I had a similar problem with the Narco 810 in my 170 when I had it. I replaced the antenna, then the coax, but neither seemed to help. Then I pulled the radio out of the rack and noticed that the connector on the back had some light corrosion on some of the pins. Cleaned them up with contact cleaner, check it first as the body of the connector may be plastic, and a small brush. The problem was solved. Just something else to check. Just as information, before I pulled the radio I connected an extension cable and a hand held com to my antenna and performance with the hand held exceed 40 miles talking to ground stations from 3000 feet. Hope this helps.

Tim

How do you utilize that alphabet-whiz-wheel? (Aren't you glad I didn't ask you anymore questions about preferred positions?)

Thank you for all the helpful comments. Especially, the watch-building types!

I do plan on removing the very old coax and installing rg400, and have a newer antenna I was already planning on installing.
I checked the ground and it seems fine.
The kx170b radio is fresh back from the shop, so I assume it's output is fine.

I should clarify: I can talk to Houston approach, if I turn 90 degrees away.
Just not when I'm pointed at Hobby tower, where their repeater antenna is located.
All the turning makes them nervous