On Sun, 01 Dec 2013 09:49:32 -0600, amdx wrote:

Think again please.
I did, a couple times.

Keep trying. Enlightenment requires suffering. Ask any philosopher.

I wondered, how does the ground (the dirt) under the radial ground
affect the pattern?

Actually, almost anything conductive or absorptive affects the antenna
pattern. In the case of "ground", I'm not talking about earth ground.
Rather, all the metal and conductive components the comprise your
house.

Then I thought: The classic pattern is not exact, otherwise I'd get no
signal in my house with the antenna on the roof.

Correct. Theory is idealized reality. To see reality, drugs like LSD
are sometimes helpful. When searching for enlightenment in antenna
design, I prefer prescription pain killers, to dull the suffering.

Also (I thinked) just changing from the 8" rubber ducky to the the 1/4
wave on the roof will make a huge difference.

That depends on what's inside the 8" rubber ducky antenna. 1
wavelength at 100 Mhz is about 118" making 8" = 0.07 wavelengths long.
That's right at the borderline where the antenna pattern falls apart
and gain starts to drop. Of course, that assumes that the rubber
ducky is properly matched to 75 ohms or is a helical antenna. What
happens with short monopole antennas (not rubber ducky antennas) is
that as the antenna shrinks, the gain remains roughly the same as a
1/4 wave monopole. With matching, the bandwidth becomes narrow. (You
can have gain, bandwidth or size... pick any two). I think rubber
ducky (end fed helical) antennas are roughly the same. However, at
0.1 wavelengths, the gain finally starts to drop. I did a crude study
of the effect on monopoles:
http://www.LearnByDestroying.com/jeffl/antennas/Monopole/index.html
The numbers in the file names are the antenna length. For example,
monopole_0_0250 is 1/4 wave long. The NEC files are suppose to be in
the NEC directory, but seems to have evaporated. I'll fix later.

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?

Yes, if you are on top of a mountain, and are stuck with antenna that
is end fed and suffers from pattern uptilt. Using a realistic model
of both the structure and the antenna, it can be demonstrated that
most of the RF is going to heating the sky and talking to birds. In
other words, little RF is going to the ground, where the mobiles and
handhelds are hiding. By inverting the antenna, usually on a tower
outrigger, the RF is redirected BELOW the horizon and more towards the
ground. Such problems are very common at higher frequencies (above
400 MHz) where vertical radiation patterns are narrow, and gains are
high.
http://www.proxim.com/products/knowledge-center/calculations/calculations-downtilt-coverage-radius
However, you're probably not on top of a mountain and do not have
enough gain for vertical radiation angle to be a problem at 100 MHz.

Darn, not an original idea!

I've never had an original idea in my life. Everything I say or do is
based on the work of others (shoulders of giants and such). The trick
to an original idea is getting away with stealing the idea, and making
it sound original. See the broken US patent system for how that
works.

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.

Second opinion perhaps? The question really boils down to how does a
tower or pipe affect the antenna pattern since it's the mounting
structure that has a bigger effect than the coax. Of course, that
assume that the antenna has a balun to prevent the coax from
radiating, which can produce all kinds of disgusting changes to the
pattern.

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.

Not much happens. The mismatch loss between 37 and 50 ohms is about
0.1dB. You can be rather sloppy with coax cables and antenna
impedances and still have a system that sorta works. I use 50 and 75
ohm coax cables almost interchangeably with problems. The problems
appear if the transmitter is unable to transmit into a mismatch and
protests by either shutting down, lowering the TX power, or going into
oscillation. all these are possible and should be tested before using
a mismatched antenna. A mismatch will also have some effect to the
antenna pattern. Where a perfect match is required are for situations
where the reflected power is capable of doing some damage (high TX
power), where you need every bit of RX sensitivity you can squeeze out
of the system (satellite work), or you simply want the very best
system. I don't think your FM BCB setup qualifies for any of these.

Ok, as you might have guessed, a lot of this is, as a previous boss of
mine, used to call "mental masturbation"

http://en.wikipedia.org/wiki/Thought_experiment
The trouble with antennas is that you can't see the RF. We use test
equipment as a blind man uses a cane. At best, you can only get a
rough impression of what is happening. There are always surprises.
Besides, it's amazing how much better things work if you think or
model before you build.

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.

I think if you simply calculate the path loss of this system, you're
going to be hurting for sufficient signal. I'm too lazy to do it
right now. Based on previous pirate radio experiences, you'll be
lucky if you get 100 ft range to a portable radio. Try a range test
with the antenna near the ground, and just walking away until the
signal gets noisy.

(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.

Heh-heh. Make me rich and a broken 8555 can be yours. I have 3 of
them but only 1 works, so this will be a repair job. I can probably
fix it, but don't have the time or incentive.
http://www.LearnByDestroying.com/jeffl/pics/home/slides/test-equip-mess.html
You'll find that the hp8554B plugin (0-1200MHz) is more useful for FM
BCB use.

A 13 element filter is going to be lossy. How lossy depends on the
design and construction. I suggest you make some measurements or just
remove the filter and see what happens.

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.

With a 90 degree vertical radiation pattern, height doesn't really
matter. Just get it closer to the receiver and you should be ok.

Jeff, at this point, I have convinced myself putting a gain antenna on
the roof will solve any problem I may have.

Are you sure? Putting it on the roof will move the receiver about
15ft further away from the antenna and add some minor coax cable
losses. That might be useful if you want to illuminate the
neighborhood, but if you're already having signal strength problems,
it's just going to make it worse. Also, do you have any idea of how
large a "gain antenna" will be at 100 MHz? You might be better of
with a dummy load and leaky coax snaked through the yard.

If not I can still remove
6db of attenuation. So unless you want some mental exercise, don't over
do it!

Mental or metal exercise? I get to help move a 2 ton mill into a
friends garage today. The exercise will not be mental. I'm suppose
to be the "safety" officer, which is a little like hiring the fox to
guard the hen house. If you don't hear from me, you can guess what
happened.

For your amusement. I'll turn it into a web page eventually:
http://www.learnbydestroying.com/jeffl/antennas/dish-move-project/
Moving and aligning big 600 lb dish antennas is easy. Polar mount
alignments are easy. Equatorial mounts are difficult. I had a little
too much fun with the cutting torch.

I think you suggested a vertical dipole with downtilt. Any thoughts
about matching, I have no clue how tolerant this little transmitter is.

Build a folded dipole out of twinlead and strips of dry pine or
plastic. Plenty of instructions on the web. Add a 300 ohm to 75 ohm
balun to match the RG-6/u. Mount it horizontally or vertically, it
doesn't matter. Position it as close to your working area as
possible. Do whatever is necessary to keep it away from conductive or
absorptive objects, such as walls, trees, towers, poles, people, and
junk. Optimizing a wide beamwidth pattern is a waste of effort.

Don't read these:
http://www.pcs-electronics.com/guide_antenna.php
http://www.radiobrandy.com/FMAntenna.html
http://www.part15.us
http://www.radiobrandy.com
They might give you some more ideas and produce more mental
masturbation.

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.

Have you measured your power output? A 100 MHz scope across a 50 ohm
load will suffice. (Remember that it's -3dB down at 100MHz). Measure
the peak to peak voltage. Divided by 2.8 to get RMS voltage. Square
that and divide by your dummy load resistance to get power.

Ummm... what problem are you trying to solve?
Who said I have a problem? ;-)

If people did not have problems to solve, nothing would ever happen.
If you need a problem, just ask, and I'll supply one for you.

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.

Define "works great". You wouldn't be asking such questions if it
works as expected.

I run my internet radio into the FM transmitter so I can listen to it
around the home.

I just turn up the volume control to get the same effect.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558