How to ensure Whip Antenna is matched to v.Low Power 100Mhz FMTX

[Xanax]

Hi All,
Does anyone know how I would match a 90cm whip antenna to a low power 100Mhz
transmitter.

I'll be taking the RF from the O/P driver transistor of the TX and through a
matching circuit to the whip antenna.

What is the usual Zo of whip antennas?

Cheers,
Xanax,

 

[John Woodgate]

What is the usual Zo of whip antennas?

It depends what you've got to act as a ground plane. In principle, a
quarter-wave unipole has a centre impedance of 37.5 ohms resistive. Your
whip is short of a quarter wave, so it will need some series inductance
to tune it.

 

[Kevin Doyle]

Great Thanks

 

[news.bluebonnet.net]

Hi All,
Does anyone know how I would match a 90cm whip antenna to a low power
100Mhz transmitter.

I'll be taking the RF from the O/P driver transistor of the TX and through
a matching circuit to the whip antenna.

What is the usual Zo of whip antennas?

Cheers,
Xanax,

Hi, Xanax -

The terminal impedance of a resonant vertical monopole (ground plane
antenna) is usually said to be half that of a half-wave dipole. So, John is
correct (of course).

Let me point out that a resonant ground plane (a whip with ground radials)
will be about 72cm at 100 MHz. So, if you use the 90cm whip, you'll have an
antenna that will have higher terminal resistance (compared to a .25 wave)
along with considerable inductive reactance.

Also, I have run tests on a (supposedly resonant) whip mounted on a
hand-held aluminum box which was about 8 x 4 x 4 inches and found the whip
feedpoint impedance to change from about 20 Ohms and reactive capacitively
up to about 50 Ohms and reactive inductively. It all depended on
orientation, height above ground, and whether the box was sitting on another
object. If your application is hand-held you may have to accept the fact
that you may not be able to provide an ideal match under all conditions.

I suggest you try rec.radio.amateur.antenna for more information.

Good luck.

John

 

[Kevin Doyle]

Great feedback lads thanks!
Xanax.

 

[Tam/WB2TT]

Hi, Xanax -

The terminal impedance of a resonant vertical monopole (ground plane
antenna) is usually said to be half that of a half-wave dipole. So, John
is correct (of course).

Let me point out that a resonant ground plane (a whip with ground radials)
will be about 72cm at 100 MHz. So, if you use the 90cm whip, you'll have
an antenna that will have higher terminal resistance (compared to a .25
wave) along with considerable inductive reactance.

As others have pointed out, the feedpoint impedance will be around 37 Ohms.
Not terribly useful. If you build it as a ground plane with a 72 cm
radiator, and four 72 cm radials, droop the radials 45 degrees downward, and
the feedpoint impedance will become 50 Ohms. You can tune the resonance by
trimming the length, and the impedance by going +/- on the 45 degrees.

Tam

 

[Kevin Doyle]

Thanks thats good info!
Cheers,
Xanax.