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Subject Topic: what transmits further? 1/4 wav or dipole Post ReplyPost New Topic
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niceguy
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Posted: 11 May 2013 at 8:09am | IP Logged Quote niceguy

hi, im wondering if somone can advise which is the best
type of antenna, and optimal height for mounting, and what
the differences are between them.

best coverage, best sound etc

Also, How much more effective is a Stacked Dipole Antenna,
over single dipole?
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RadioTech
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Posted: 14 May 2014 at 2:42am | IP Logged Quote RadioTech

Sorry for the delay, it's a year on but this may still be of use to you or for someone else who might be reading.

With regards to height, when it comes to FM broadcast, height is might. The higher you can get the antenna, the better the coverage you will have.

As for 1/4 wave and dipole antennae, the simple answer is "it depends where you plan to transmit from".
If you plan to transmit from your house in the middle of town, the 1/4 wave will give you equal radiation in all directions.
If you plan to transmit from a hill just out of town, the dipole is more suited as it focuses the radiated energy more in one direction.
That's the easy way to answer without getting into a lot of deep theory on antennae.

As far as sound goes, you would not hear any difference between the two.

As for stacked dipoles, this is done for several technical reasons but the main effect is an increase in gain so you must factor this in to your calculations when setting up transmitter power etc.
This obvoiusly increases the radiation into a defined area but it has to be set up accurately or you can end up with the opposite effect and have deep nulls in your radiated pattern.
For LPFM, stacked dipoles simply aren't necessary.

I hope this has helped. If you have any further questions ask away and I'll do my best to answer them a bit quicker than I got to this one!
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Esprit
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Posted: 25 October 2016 at 1:26am | IP Logged Quote Esprit

Hope you do not mind me commenting on the question.

If you look at the radiation pattern of a ground plane
antenna, you will find that the maximum radiation lobe is
usually about 20 or 30 degrees above the horizon, making
it a compromise if you are transmitting from a high point

With a dipole, the radiation lobe is to the horizon, but
when a dipole is mounted alongside a metal mounting pole
or mast, the pole or mast can cause less radiation behind
the mast than in front of it.

My preference would be to construct a coaxial dipole.
This is very similar to a ground plane antenna but where
the radials have been replaced with a sleeve of the
correct length and inside this is the mounting pipe that
extends below the sleeve. This antenna on top of a pole
or mast radiates with its lobe to the horizon.

I am sure if you google coaxial dipole you should find
construction details.

__________________
Active radio amateur from 1970 through to 2003.
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traderbob
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Posted: 11 September 2017 at 4:05am | IP Logged Quote traderbob

LPFM aerials are very tricky to get right.
While a high point on a hill is a great place for coverage, the signal you deliver to where the people live will be attenuated by distance, and it's a logarithmic factor. The greater the distance from the top of the hill to town the less you get per km by an ever increasing amount. LPFM signal degrades to poor quality between 3km and 5km. The line of sight path from that hill to the target area can easily be 5km.
The big guys use stacked arrays of 2 4 or more dipoles and 'tilt' to beam the signal down.
Best antenna placement is just above roof height in the centre of your service area. The aerials to use are J Pole or Dominator which both have vertical polarisation and both have a very good sideways radiation pattern. The dominator is bette because it has a better bandwidth than a typical J pole although you can make a jumbo j pole which works well. Use LMR400 coax or RG213 and keep it as short as possible. LPFM will give a 3km service area maybe 5km to domestic FM radios.
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