antenna problems

[tangocharlie123]

Hello All

 

I'm after info on this antenna instalation the antenna is a comant fiberglass whip Don't know exact model

 

The antenna is located forward of the aluminium ground plane. should I locate this to the middle of the ground plane.

 

the problem that I have is unless I am on the ground or in the circut or within a few miles of the airfield I don't belive that I am bieng heard clearly.

 

the other thing is that when I climb to 1500asl I can finnaly hear bn centre and flight watch.

 

microair 760 early model is the radio.

 

thanks for your help

 

Peter

 

 

 

[jetboy]

Peter,

 

The antenna appears to be mounted on a strong location (good) but without any load spreading doubler plate (poor) and the groundplane should be centered on the antenna mount whether the antenna is a sloping one or not.

 

Could you bend up a doubler .040 or .063 aluminum and extend it over the ridge to join the existig groundplane with a line of rivets (pry the groundplane away a bit to do this without holes thru the fuse?

 

Check that the comnant grounds thru the bolts and discard the earthing wires.

 

If you have area forward of the antenna, no need for the aft groundplane at all, just run four radials of copper tape or wire braids in an X pattern from the bolts, the radials should be 24" long each. you could use strips of Al but i wouldnt use Al tape unless your careful connecting it, as it will come away later or corrode at the bolts.

 

Ralph

 

 

[Guest Andys@coffs]

Might be just me, but the wires look to be normal wire not 50 ohm coaxial cable. If it is just normal wire then that almost certainly is the issue. I would also ask if a VSWR meter reading was made at the time teh antenna was commisioned, against which a reading could be compared.

 

If the wire is plain wire and not coaxial cable then the VSWR is likely to be very poor and at the root of the entire issue.

 

Andy

 

P.S ideally the antenna should sit in the middle of the ground plane, however the reality is that with the 5-6W that a 760 generates you should be able to have a sub optimal antenna / ground plane solution and still get miles and miles of performance.

 

p.p.s The ground plane is called the ground plane for a reason.....It must be grounded to Aircraft electrical earth. Only do away with the Earth wire if indeed the metal ground plane is earthed via some other alternate mechanism.(ie the earth braid of the coax etc)

 

 

[tangocharlie123]

thankyou Guys

 

Andy it is coax but the wire from the antenna base to the ground plane is normal wire

 

my thoughts would be to move the antenna but after alternatives without putting holes in the plane.

 

or just purchase a new internal antenna like whats in the jabiru.

 

Thanks

 

Peter

 

 

[Yenn]

Does anyone know of a cheap source for a SWR meter?

 

 

[Admin]

Does anyone know of a cheap source for a SWR meter?

I am willing to bet Dick Smith has them

 

 

[Smokey]

I am willing to bet Dick Smith has them

You're a brave man suggesting that Ian, since ds got taken over by woolies they've been steadily dropping all links back to the store's roots and replacing it with consumer electronics. All woolies wanted was the name.

 

In fact, out of curiosity I just did a quick search of the website and no swr meters came to light.

 

Yenn

 

Be careful about getting hold of a 'cheap' swr meter. check what frequencies it's designed for, most of the cheap ones were made for the old 27MHz CB radios, these could give misleading numbers at 120+MHz where we work.

 

If noone at the airfield has a suitable meter you could always try ham radio operators, they will usually have a multi-band swr meter that covers airband and I'm sure they'd looove to help you out with an excursion to an airfield, especially if there's the prospect of a hop. They'll probably also be able to look at your installation, just remember, like us they are amateurs and the knowledge of any individual could be anywhere from non-existant to encyclopiadic (is that a word?). You'll find ham clubs listed on the Wireless Institute of Australia website (www.wia.org.au).

 

If you particularly want to purchase one, check the ham radio mags for ham radio dealers, they should be able to source one that will suit without going to a unit with unnecessary bells and whistles.

 

Mark

 

 

[Yenn]

Thanks Smokey. I had been looking in the past and was aware of the different frequencies.

 

I am fitting an Icom A200 into the Corby so not much chance of giving a ride. Emailing the nearest Icom agent didn't even get a reply when I was getting quotes to but the radio. It looks as if I will just use the theoretical length aerial which jas worked with a handheld.

 

 

[sain]

You can get a suitable VHF SWR meter here:

 

(http://www.radiospecialists.com.au/Power%20meters.htm the DH2475 looks suitable but will set you back about $100).

 

Another thing you might want to check out on that antenna installation is the connectors on the coax itself - if they are broken you'll get ****ty reception/transmit

 

 

[Guest airsick]

Michael Coates also sells them (I think). www.mcp.com.au

 

 

[tangocharlie123]

update

 

Just to let you know

 

Yesterday I was out at the plane doing some work I opened the rear cover where the antenna base is and knocked the coax, low and behold it fell off. reattached it properly.

 

Don't know how it stayed on with all the landings.

 

The reception is better but will take it up today to see what sort of range.

 

I am still considering moving the antenna but will see.

 

Thanks for the advice :thumb_up:

 

Peter

 

 

[Admin]

:faint:good to hear

[sain]

Don't forget to check the other end - if its bad at one, the connector to the radio might be dodgy too.

 

 

[Ross]

Hi Peter

 

I just looked at your pic and it appears that you have a fibreglass aeroplane and unless it is covered in metal it occurs to me that it might be easier to install an internal dipole aerial if you have the vertical space.

 

I just ducked out to my garage to get an approximate total length from top to bottom of the aluminium dipole aerial installed in the tail of my J160 kit.

 

I think dipole aerials have a better rating than 1/4 wave aerials and do not waste as much power by radiating a signal basically in a ball shape.

 

A dipole aerial basically radiates most of its energy out horizontally - so there is an advantage in mounting it so that it stands vertical in the ac when in flight although it operates OK in the Jabiru when mounted in the back of the vertical stabiliser which is on a small angle to the vertical.

 

A dipole aerial does not need a ground plane and consists of two legs with one connected to the core wire of the coaxial cable and the other leg to the outer sheath of the coaxial cable.

 

Jabiru specify which wire to connect to the top element of the dipole and which coaxial part to connect to the bottom element of the dipole.

 

They are normally not connected electrically but I think they can be provided a suitable resistance is in the circuit.

 

So normally a dipole aerial cannot be tested for continuity from the radio end unless a single extra wire is run from there back to each of the dipole elements during the test with the coaxial not connected to the radio.

 

I think there was an article on this subject in a Pacific Flyer magazine quite some time ago.

 

If they were connected electrically they can be tested for continuity with a multimeter as you can do from the radio end with a multimeter on the disconnected coaxial at the radio end of a coaxial cable connected to a normal 1/4 wave aerial with a ground plane..

 

The total dipole is slightly less than 980 mm overall length with a gap in the middle of 12 mm I think.

 

The middle gap is critical as is the length of each element of the dipole aerial which is just two thin strips of aluminium on my ac.

 

I could get more accurate measurements in daylight if you want them and check out the specified gap from the Jabiru construction manual.

 

This aerial was set up for the early radios in the jab which did not have the weather frequencies so there might be a slight difference in an aerial to handle the ATIS frequencies as well. I do not know.

 

Regards