antenna

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ed
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antenna

Post by ed » Thu Jan 10, 2019 9:05 am

I'm a numpty with radio and having trouble finding info for wiring coax to a receiver.

At the moment I use a(tuned 1/4 wave) length of copper wire as a whip aerial, attached 1" from the receiver. I want to have a 1/2 wave (off the shelf)to maximise the range but it has to be located away from the receiver so has a length of coax built in.

question is: do I connect the coax sheath to ground at the receiver end?

I've so far failed to find this info on the web.
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Re: antenna

Post by Nick » Thu Jan 10, 2019 11:25 am

Is it a dipole? Then yes,

else

My gut feeling is yes, but as to if it matters much depends on if its got a built in ground plane. Also without the screen being connected, the connecting wire will be transmitting as well.
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Re: antenna

Post by ed » Thu Jan 10, 2019 12:42 pm

it's a monopole

my only thought was intuitive, to wit, if the connector acts as a receiver then i lose any of the tuning benefit, i.e it ceases to be a 1/2 wave antenna......

there seems to be no empirical feedback for this on the net...

here's an example:
https://www.ebay.co.uk/itm/433MHz-Anten ... SwKQFbpOfl

at the moment I just connect a 165ish mm wire to the antenna connection on the receiver...there is no help on the receiver datasheet as it probably assumes the user knows what they are doing........problem is I am using this at it's extreme transmission range so I need to be as close to optimal as I can.
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Re: antenna

Post by Nick » Thu Jan 10, 2019 6:04 pm

my only thought was intuitive, to wit, if the connector acts as a receiver then i lose any of the tuning benefit, i.e it ceases to be a 1/2 wave antenna......
I assume you mean transmitter there, if so, yes I agree.
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Re: antenna

Post by Mike H » Thu Jan 10, 2019 8:29 pm

ed wrote:
Thu Jan 10, 2019 12:42 pm
my only thought was intuitive, to wit, if the connector acts as a receiver then i lose any of the tuning benefit, i.e it ceases to be a 1/2 wave antenna......
I've done this, what you want to do, and apparently not, the antenna still works like a dipole despite one side being connectected to receiver chassis via a coax screen. This was for FM.

I made one earlier but cocked up, I got the idea in my head I need 2 metres for FM, but no it's 3 metres. That explains why it don't work too well at the mo, so now I'm planning a 1/2 wavelength (1.5 metres tip to tip), also I'm going to use two telescopic antennas which can then be precisely adjusted for length. Should be 75 cm each. (Have done this before and worked quite well.)

HTH

NB: if on the other hand you're doing an AM aerial, then it's a bit different, basically it's a long wire and the earth (literally, the earth) is one half of the antenna. Ergo your radio chassis needs to be mains earthed or, as in the old days before 3-pin plugs ( :D ), you had a wire going out to a metal spike in the ground outdoors. This is if you want best sensitivity. (NB: I've done this too. Had a long wire in the attic and earthed the radio thru the mains earth. Which occurs naturally anyway cos of having a 3-core mains lead.)
Last edited by Mike H on Thu Jan 10, 2019 8:41 pm, edited 1 time in total.
 
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Re: antenna

Post by Mike H » Thu Jan 10, 2019 8:39 pm

PS: the coax cable will behave like a transmision line, which is why it won't behave the way you might be thinking. :wink:

Also needs to be terminated with the correct input impedance at the receiver, e.g. 75 Ω for FM.
 
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Re: antenna

Post by Nick » Thu Jan 10, 2019 8:59 pm

Mike H wrote:
Thu Jan 10, 2019 8:39 pm
PS: the coax cable will behave like a transmision line, which is why it won't behave the way you might be thinking. :wink:
But only is the screen is connected,
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Re: antenna

Post by ed » Thu Jan 10, 2019 9:29 pm

I'm not sure if we are going off the path here, but in truth I don't really understand what you're both getting at here...

My receiver is am, 433 mhz and this guy is doing exactly what I have been doing for some time:

https://www.youtube.com/watch?v=v9JMZyeTkKQ

the question arises because I want to go to 1/2 wave receiver aerial and it has to be a short distance from the receiver.....now the ready-made antennas I'm looking at have coax factory fitted to the antenna unit, as shown in my earlier post......

the question is: do I attach the coax screen to ground at the receiver end?
simply because if I don't I thought the middle of the length of coax would become part of the antenna and therefore destroy any tuning that I had due to the exact length of the antenna itself.

I apologise if my descriptions are not very precise.
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Re: antenna

Post by Nick » Fri Jan 11, 2019 8:57 am

Sorry my fault, thought you were transmitting (not that it makes much difference). Anyway, I agree with you, connect the shield.
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Re: antenna

Post by Mike H » Fri Jan 11, 2019 3:44 pm

Image
 
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