I blame my poor memory :-)

I've checked with the experts, a standard dipole has a nominal gain of
2.2dBi.
But the principles are the same.

I always forget the exact figures for dBi - mainly because in my
professional life I always deal with gains of between 6 and 10dBi on our
large antenna systems

I've just been looking at a real-world example:
Antenna gain 4.2dBi
Feeder loss 2.0dB
And that's for a stacked dipole array offset from the pole.

Cheers, Jochen

>>> brianislay@... 8/02/05 >>>
Er Jochen,
Since when has a dipole antenna had 3db gain?
I am assuming a 1/2 wave ant.
Cheers
Brian

---------------------------------
Do I?

OK, here's how it works.

First the golden rule: Everything in FM works
backwards.

Why?

Receive level dictaes radiated power which dictates
antenna gain which
dictates antenna height wihich dictates feeder loss
which dictates
transmitter size.
Roughly.

For LPFM, the radiated power has a permitted maximum
of 500mW e.i.r.p
(effective isotropic radiated power) which is -3dBW.
The maximum is set
by the GURL licence for LPFM.
You could radiate less, but what's the point?

So to achieve the maximum of 500mW out of the antenna,
you need to know
your antenna gain and your feeder loss. This will then
dictate your
maximum permitted transmitter power.

But for flee-power LPFM space, don't bust a boiler
trying to be too
accurate.

There's not much point in using anything other than a
dipole or a
j-pole antenna - extra antenna gain gives no benefit
in the LPFM space.
In the high power space, antenna gain is used to keep
transmitter
powers and electricity consumption down. Both have a
big impact on
operating costs, but for LPFM, transmitters are cheap
and operating
costs are peanuts.

So use a dipole (approx 3dB gain) and some coax
cable.
J-poles are good if you want to ground the antenna to
provide extra
lightening protection - a good idea, as I know of some
Veronica
transmitters that used to die regularly on a dipole,
and were reliable
on a J-pole due to the better protection.

Max licence power of -3dBW less 3dB antenna gain means
you feed the
antenna with -6dBW of power. That's 0.3 watts into the
antenna connector
at the antenna.

Assume your coax has 6dB of signal loss because it is
nice and long, so
you get -6dBW + 6 = 0dBW to be fed into the end of the
antenna coax.

And guess what 0dBW equates to?

You got it .... 1 watt.

If your coax feeder loss is less than 6dB, you will
need to reduce your
transmitter power to stay within the regulations. This
has the added
benefit of reducing the hard work that the final
amplifier has to do,
and making it more reliable.

Cheers, Jochen

>>> mixfm@... 7/02/05 >>>

Yes, but when you take into account loss due to SWR
(which are affected

by a number of factors), loss through co-ax cable, the
fat pigeon that

roosts on your antenna, etc, it's not hard to take you
down to around
0.5 W.

Jochen usually replies to these types of emails.....

Michael Rowse wrote:
> Isn't 1 watt twice what you are allowed under the
rules of LPFM here
in NZ??
>
> I'm just checking because I'm about to buy a TX too
and 1 watt would
be
> great...much better than the 500mW that I thought we
had to have.
>
> */Jo Hodge <j.c.hodge.bro@...>/* wrote:
>
> Joe Hodge Wanganui,
> Music Memories FM 88.6, Wangani Community Based
Radio Staion for
> Senior Citizens
> We are a charitable natured organisation.
>
> A big Hi to the LPFM group.
>
> Does anyone out there have a Tx to sell.
Preferably NRG 1watt.
> Hope to hear from somebody soon.
>
> so36jo@... <mailto:so36jo@...>
> phone:(06) 347 8143
>
> Bye.
>
>
>
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>
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