El 01-12-13 16:49, amdx escribió:
On 11/30/2013 10:30 PM, Jeff Liebermann wrote:
On Sat, 30 Nov 2013 21:42:38 -0600, amdx wrote:

I have a low power FM transmitter that I use for in my house and yard.
I want to put a 1/4 wave vertical on the roof of my home. The 1/4 wave
vertical will be made as many have seen from a UHF panel mount
connector
with the vertical on the center pin and the four radials soldered
to the
holes for the screws.
Like this,
https://encrypted-tbn1.gstatic.com/i...M00RrltFyCH-LA

Only bigger :-)

Unless you use stiff elements, they will sag. At 100MHz, 1/4
wavelength is about 75 cm (2.5 ft) long.

Looking at the pattern of a 1/4 wave vertical, I think I could best
cover my yard with the pattern upside down.

Think again please.

I did, a couple times.
I wondered, how does the ground (the dirt) under the radial ground
affect the pattern?
Then I thought: The classic pattern is not exact, otherwise I'd get no
signal in my house with the antenna on the roof.
Also (I thinked) just changing from the 8" rubber ducky to the the 1/4
wave on the roof will make a huge difference.

Can I mount the antenna upside down?

Yes.
Will I knew I could! We have an upside down building not far from here.
The better question, is there a good reason to mount the antenna
upside down?

Is this feasible?

Yes. It's commonly done with UHF antennas on mountain top sites.

Darn, not an original idea!


If I did turn it upside down, what would the feedline do to the
pattern?

The feed line will mangle the pattern.

So why did I ask, I knew that.


Is there a better physical layout to avoid pattern distortion caused
by the feedline?

Yes. Use a vertical dipole on a tower or a coaxial antenna on a vent
pipe.

Hmm, vertical dipole,
But, then I'd miss the excitement of paralleling 90* of two 75 ohm
coax cables and the measurements to match 37 ohms to 50 ohms. I wanted
to see that happen. I guess I could still do the experiment.

The alleged problem with a ground plane antenna is that there is a
slight vertical uptilt of the beam. It varies with the height above
the rooftop ground, but my guess(tm) is maybe 5 to 10 degrees uptilt.
I just ran a simple ground plane simulation using 4NEC2 and found that
the uptilt is small when the vertical beamwidth of the ground plane is
about 90 degrees. In other words, inverting the antenna isn't going
to do much good at delivering the signal towards the ground. You're
better off with an antenna that puts the main lobes where your
receiver is located or perhaps has some gain and/or downtilt. Without
a description of your house and yard, I can't offer any suggestions.
Numbers please?

Ok, as you might have guessed, a lot of this is, as a previous boss of
mine, used to call "mental masturbation"
I do intend to mount an antenna outside (again), now, I'm not sure
what type.
The problem I'm solving is, in some areas of my yard, the radio signal
gets buzzy, sometimes turning the radio will fix it, often I have to
move the radio 5ft to get a clear signal.
My transmitter is a CZH-05B, the power is switchable between 0.1 watt
and 0.5 watts. I run it at 0.1 watt in an effort to keep myself out of
trouble. Hmm, as I'm writing I noted I have two 3.0db attenuators
before the antenna, as more keep myself out of trouble units.
(Btw, I just modified a 13 element filter to put between the
transmitter and the antenna. Someday I hope to have the equipment to
analyze it and see how well it works ( how well I did). I started with
a TFD6102A and wound new coils and added capacitance as needed. I have
not installed it yet.) I recently bought an HP 141T/8553, if I find a
8555 at a reasonable price, I'll buy it.
The receive area is small, 120" x 115", the antenna will be mounted
13" in from the long dimension and 16" in from the smaller dimension,
basically in the corner of the lot, mounted 16 ft high.

You mentioned 0.1 W with 2, 3 dB attenuators, so you may have around
10..20mW at the antenna (depending on cable loss). This should be more
then enough to cover your yard if you find some clear frequency.

I would just try the quarter wave antenna with three or 4 radials on
the roof (not up side down). Due to scattering on the roof, and may be
some common mode current on the feed line, you will have sufficient
signal indoors. If you live in shielded room or heavy bunker/shelter,
you may not have indoor coverage.

You may give us some height and lot size info.



Jeff, at this point, I have convinced myself putting a gain antenna on
the roof will solve any problem I may have. If not I can still remove
6db of attenuation. So unless you want some mental exercise, don't
over do it!
I think you suggested a vertical dipole with downtilt. Any thoughts
about matching, I have no clue how tolerant this little transmitter is.
Hmm, maybe put a 3db attenuator on the transmitter output, into the
LPF, then another 3db attenuator between the LPF and the antenna.
Then my transmitter's happy, and my filter is happy.


You might find it useful to look at what the LPFM people are doing for
antennas:
https://www.google.com/search?q=lpfm+antenna&tbm=isch
Remember, the stranger it looks, the better it works.

Ummm... what problem are you trying to solve?

Who said I have a problem? ;-)

I'll add, I have messed with the Ramsey FM transmitter and a couple of
others, This transmitter works great, and they're down to about $60
now, half what I paid two years ago. The newer models go up to 7 watts,
if you can believe the specs.
I run my internet radio into the FM transmitter so I can listen to it
around the home.

Mikek



Best regards,

--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM