On Mar 2, 10:40 am, "MRW" wrote:
Good morning!

I would like to request some additional info regarding the following
matter.

Is there a way to approximate the antenna factor loss (in dB/m) for an
x-length (in inches ... maybe 26 inches) regular telescoping, monopole
antenna that will be subjected to frequencies between 88-108MHz (FM
band) at a height between 2ft to 15ft from the ground?

Here's the scenario. A 50-ohm receiver will be used. A cable with an
insertion loss of 1.5dB will also be used. The receiver has
sensitivity levels as low as -5 dBuV. The lowest received signal that
we want to measure is 100uV/m.

I am concerned that if an unmatched monopole antenna is used in this
system, then it is possible that very low level signals will not be
measured. I'm trying to estimate the lowest amount of signal the
receiver will be able to pickup using this setup.

I have the following relationship:

dB(uV/m) = dBuV + Antenna Factor (dB/m) + cable loss (dB)

So, I get:

dBuV = 40 dB(uV/m) - Antenna Factor (dB/m) - 1.5 dB(cable loss)

Now, if only I can estimate the antenna factor, then it would give me
an idea whether or not the resulting dBuV level is still higher than
-5 dBuV.

I was told that there may be a way to estimate the antenna impedance
assuming certain conditions. I was hoping to find some reference for
this subject, and just assume how much of the signal is lost due to
the antenna impedance and 50-ohm receiver impedance mismatch. I have
seen some nomograph for VSWR, so I figured if I can find the estimated
antenna impedance at the given conditions, then I can calculate
reflection coefficient and relate that to VSWR and then to the
nomograph to get my antenna factor value.

What do you experienced ones think? Thanks!

I believe you are going to be more concerned with the transmission
line loss and the additional loss incurred due to the mismatch if the
mismatch is severe.

To obtain a desktop estimate of the VSWR, download W7EL's EZNEC demo
version (www.eznec.com) and put in the antenna dimensions, frequency
of interest etc. It will calculate the impedance and VSWR for you.

Take those values and go to http://www.vk1od.net/tl/tllc.php and plug
in your values along with the cable you plan on using.

You can also pull the ARRL Antenna book and look for the formula to
calculate the additional transmission line loss due to a load
mismatch.

Harry

"harry" wrote in
ups.com:

....
I believe you are going to be more concerned with the transmission
line loss and the additional loss incurred due to the mismatch if the
mismatch is severe.

Harry,

At the risk of reawakening the source end re-reflection noise, this is a
receiving application, if the receiver is 50 ohms (as the OP stated) and
50 ohm coax is used to connect to a short monopole over ideal ground, the
VSWR on the line is 1:1 (or near enough), and loss is the matched line
loss.

The implications of that are that the short monopole, which has a source
impedance of very small radiation resistance and loss resistance, and a
huge capacitive reactance is loaded by the 50 ohm impedance looking into
the coax.

You can relatively easily work out the o/c voltage in the whip as a
result of an incident E field (it will be ~E*length/2), but you must
estimate the capacitive reactance as it is the main determinant of the
voltage divider formed by the antenna source impedance and the 50 ohm
load.


To obtain a desktop estimate of the VSWR, download W7EL's EZNEC demo
version (www.eznec.com) and put in the antenna dimensions, frequency
of interest etc. It will calculate the impedance and VSWR for you.

The impedance from NEC (of any flavour) can be used to workout the
voltage divider I mentioned above.

Alternatively, use a variety of NEC where you can excite the antenna with
an incident wave and observe the voltage developed on a load inserted in
the model. The model described
http://www.vk1od.net/SmallUntunedSquareLoop/ssulNEC.htm at does such a
thing for a loop, you could do something similar for a short monopole.

My thinking is that a short monopole is less attractive at these
frequencies than using a dipole of known gain and impedance, a dipole is
convenient (probably more than a short monopole with a large ground
plane) and more sensitive (ie lower Antenna Factor).


Take those values and go to http://www.vk1od.net/tl/tllc.php and plug
in your values along with the cable you plan on using.

You can also pull the ARRL Antenna book and look for the formula to
calculate the additional transmission line loss due to a load
mismatch.

The formula in this case for the monopole termination loss is 20*log(50/
(Zs+50)) and must be factored into the ratio of E field to o/c voltage to
get AF.

Owen

Thank you for the responses everyone! I will look at the links posted
above.

Owen, great site! I have added it to my bookmarks. Thank you, sir!