How to 2 antennas for better coveraged one on top one under

[Tony2]

The problem poor coverage at times * ie anytime the antenna does have any metal obstructions

 

Aircraft all Aluminum STOL

 

Antenna been checked with swr* all fittings cable has been replaced

 

Radio Yaesu 220 same problem Microair

 

It transmits and receive well when inline

 

The idea keep the whip antenna on top and use the existing rubber ducky underneath or a bent vhf

 

What is needed , what other alternatives

 

Thanks everyone in advance as this is a safety problem

 

:confused:

 

 

[jetboy]

Hi Tony,

 

Your'e not telling enough of the story; like exactly what plane this is, why it sometimes does OK and other times not. What is the SWR for 118.0, 128.0 and 136.0 frequencies? I ask this because one reading is not enough to meet the spec and normal whip antennas seldom meet it first time.

 

I'll start by stating that attempting multiple antennas is not the way to go, and rubber duck antennas are unsuitable if you are looking to have any sort of range. A bent whip (when the bend is at least further out than half its length) is OK.

 

I have a CH701, bent whip on top, 5 watt radio and it works to over 50 NM from 1500' in all directions. I am a radio tech and have field strength tested some aircraft (not this one) by walking around them with a meter to confirm the radiation pattern. With the aircraft you described I can't imagine any problem, assume the top whip is a metre away from anything else - other antenna - ELT - rudder ?

 

If the problem is reception in certain directions, there could be issue with interference from engine - strobes - HID or LED lamps ?

 

Ralph

 

 

[Tony2]

Hi Ralph

 

Just the right man to pick, it is a Savannah so same dog and can receive 50 NM away the real problem is circuits/ close to the airstrip group to air/air to ground and the SWR was done by an Communications guy and passed it as perfect as one can be.

 

No interference problems

 

I think it is this one

 

http://www.mobileone.com.au/antennas/airband/airband.html

 

M124-1 515mm Fibreglass 1/4 wave antenna suitable for all air band applications on a 5/16" fitting

 

 

Also I noticed this next one kinda what I was talking about, as I have very little knowledge on antennas thanks again.

 

 

 

 

[jetboy]

If people cant hear you in the circuit its more likely a headset / radio problem

 

Some headsets overload the mic input to radios, this can be adjusted on the Microair by someone with a modulation meter. This was the problem on the Savannah + Microair I fixed here, local club wouldn't allow it to fly, gave readability 2 reports, radio adjusted lower gain (much lower, like 50% modulation on strong voice as in riding behind a Rotax at full noise), readability now 5 and cleared to fly. Also these radios do not discriminate between pilot and pax mic sockets, flying solo best disconnect the pax headset, this has caused many "radio faults" around here.

 

I wont recommend any of the antennas pictured they wont be secure or approved for airbourne use and the Aircraft Spruce agents can get you much better antennas for the job. If your present antenna is "good" for SWR then there is no need to change it (but make sure it wont fall off)

 

Ralph

 

 

[Guest basscheffers]

Can you do this? I would imagine with the distance between the antennae would cause losses due to phase difference.

 

The wavelength of 130MHz is 230cm. So if you put them a meter apart, depending on the location and distance of the station, you could see dramatic losses with the reception on one antenna cancelling out the other because of the phase shift.

 

The proper way to solve this is with a diversity receiver; which is what the multiple antenae on a radio-mic receiver (as in live music) or your wireless ADSL router are.

 

They constantly check which antenna has the stronger signal and switch between them. Do these exist in aviation?

 

Obviously, this only works for receiving; you can only send on one antenna at a time for the same reason.

 

 

[Tony2]

Good call, I will bypass the intercom and see how I go two different brand headsets same result over modulation via the intercom.

 

Can you explain

 

I wont recommend any of the antennas pictured they wont be secure or approved for airbourne use

as the unit came from M Coates.Found this on the net interesting the part 3 and 4 The location of the antenna is of primary importance. When selecting a mounting position, consideration should be given but not limited to the following: (1) Obstruction to signal reception by aircraft or aircraft components.

 

(2) Ignition noise (RF radiation pickup).

 

(3) Vibration.

 

(4) Flutter.

 

(5) Instrument static source interference.

 

Where do recommend is the best place to mount the antenna?

 

Thanks again

 

Tony

 

 

[jetboy]

Reply to Bascheffers: Yes you can combine antennas, fit diversity receivers, its just not going to help when the problem is occuring in the circuit, My old job at an HF radio station we routinely used triple diversity systems to haul in signals, the problem on a mobile situation is any phasing of antennas will result in main beams and deep nulls in certain directions, so its not the go for Tony

 

"Can you explain

 

I wont recommend any of the antennas pictured they wont be secure or approved for airbourne use" .

 

Those antennas just have a coarse thread mounting boss and can come loose and depart an aircraft. If the aircraft is a pusher or a helicopter the effects on safety are more than trivial. If you are already using one, I'd suggest drilling and using lockwire.

 

My VHF antenna is mounted with a doubler plate running aft of the baggage space in the centre of the roof (the doubler was already there courtesy of CZAW)

 

The antenna I used was ACS part 11-04516 and cost about $25 however they are a bit heavy for the 701- I ended up welding a smaller diameter SS whip section into it and If I was doing it again would probably use the much better D&M C63-1/A part 11-02764 (ouch- the price seems to have gone up since I last got one). These properly compensated broadband antennas are more likely to work first time and have connections suitable for the correct cabling which generally saves installation time and rework.

 

Ralph