Self-building my first dipole antenna

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stev
Posts: 2
Joined: Sat Dec 19, 2020 8:24 pm

Self-building my first dipole antenna

Post by stev » Tue Dec 22, 2020 3:47 pm

I'm looking into building a dipole antenna for only 27Mhz, The antenna will be used for receiving only with an RTL-SDR v3 and, at least initially will be placed indoors.
Could you please help me clarify the following (very basic) doubts?

It seems to me from various reading that building a dipole essentially amounts to getting a coax cable and a couple of wires, connecting the ground of the cable to one wire, and the positive to of the coax to another. Cut the wires to the appropriate lengths and it's done.... Is there more to it?
I have seen mentions of a balun, do I need one? If so how do I go about building it?

Which coax cable should I choose? Which wires should I choose? Should the wires have isolation around them? What are the specs I should be paying attention to?

I have seen sources claim that an horizontal dipole has an impedance of 75 Ohms and other that it has an impedance of 50 Ohms. Which one is it? And should I match this to to 50 Ohms of the receiver? If so, how is this done?

Wouldn't an horizontal dipole immediately cause a big attenuation of the received signal since most transmissions are vertically polarized? If so, why are there many sources recommending it? Wouldn't the best orientation be vertical?

Is it possible to bend the wires back instead of cutting it, so to change the antenna frequency? If so, is the act of bending them sufficient or should they make contact with themselves? If they should make contact, is only the tip required or should it be for the whole length of the bent-back portion?

If the only electrical connections are what I've described about what' preventing the coax cable from "becoming part of" the antenna? Where should the wires of two legs of the dipole measured from? I was assuming from the point they're connected to the coax cable but I don't see what'd be different between those wires and the ones in the coax itself.

Thank you very much and sorry for the basic questions.

snn47
Posts: 253
Joined: Tue Dec 27, 2016 11:00 pm

Re: Self-building my first dipole antenna

Post by snn47 » Tue Dec 22, 2020 6:44 pm

Have you looked at "antenna related information and guidance for asking questions to improve chances for an answer"?
In addition coaxcable- Balauns and Ferrite-Baluns there are other versions to achive balanced to unbalanced feedin, just check amateur radio websites for building a 10 m dipole and increase the dipole length for the frequency you want to receive.
I'm looking into building a
horizontal
dipole antenna for only 27Mhz, The antenna will be used for receiving only with an RTL-SDR v3 and, at least initially will be placed indoors.
Could you please help me clarify the following (very basic) doubts?

It seems to me from various reading that building a dipole essentially amounts to getting a coax cable
a Balaun
and a couple of wires, connecting the ground of the cable
via Balaun
to one wire, and the
shielded center wires
of the coax to another. Cut the wires to the appropriate lengths and it's done.... Is there more to it?
No
I have seen mentions of a balun, do I need one? If so how do I go about building it?

Which coax cable should I choose?
depends on length of coayx and attenuation you will accept
Which wires should I choose?
isolated cooper to avoid corosion
Should the wires have isolation around them? What are the specs I should be paying attention to?

I have seen sources claim that an horizontal dipole has an impedance of 75 Ohms and other that it has an impedance of 50 Ohms. Which one is it? And should I match this to to 50 Ohms of the receiver? If so, how is this done?

Wouldn't an horizontal dipole immediately cause a big attenuation of the received signal since most transmissions are vertically polarized?
cross polarization will cause loss, but directivity of dipole will too
If so, why are there many sources recommending it?
to receive horizontal polarized signals without cross polarization loss
Wouldn't the best orientation be vertical?
only if you want to receive vertical polarized signals

Is it possible to bend the wires back instead of cutting it, so to change the antenna frequency?
bending won't change electrical length of wire
If so, is the act of bending them sufficient or should they make contact with themselves?
try it, because resonance depends on many factors
If they should make contact, is only the tip required or should it be for the whole length of the bent-back portion?

If the only electrical connections are what I've described about what' preventing the coax cable from "becoming part of" the antenna?
Balaun
Where should the wires of two legs of the dipole measured from?
Balaun connectors
I was assuming from the point they're connected to the coax cable but I don't see what'd be different between those wires and the ones in the coax itself.

Thank you very much and sorry for the basic questions.

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