Effect Of Antenna Height On FM Receiving Antenna Gain

[Ben Zonia]

As stated elsewhere, FM receiving antenna heights can have a dramatic effect on signal strength to your receiver. Brian Beezley from the k6sti ham radio site demonstates this with a graph. It would have a similar effect on VHF and UHF TV signals. Soil types also affect this, as do various polarizations of the receiving antenna.

Note that this is noise free gain, whereas signal amplifiers add noise to the signal.

As an example, heavy clay shows a dramatic increase of 13 dB from 6 feet AGL to 30 feet AGL (-18 dB to -5 dB for Horizontal Receiving Antenna Polarization). With a 10 Element FM Yagi type antenna with 7 dBd gain, this would increase the gain by 20 dB over an indoor dipole antenna, or 10 times the signal voltage to the antenna input on your receiver or tuner.

http://ham-radio.com/k6sti/height4.gif

[Rich F.]

As stated elsewhere, FM receiving antenna heights can have a dramatic effect on signal strength to your receiver. ...

However a considerable amount of the realized system gain in this situation is due to the increase in the transmitted E-field vs. elevation AGL at the receive site, rather than as a property of the receive antenna and its polarization/elevation.

[Ben Zonia]

I would imagine that as you get close to line of sight, there would be a rapid increase in signal, and an increase as the Fresnel Zone becomes unobstructed, and also scattering and diffraction would increase with increasing height due to different angles of departure and arrival, when the signal is not line of sight. You can see that on FM Fool as you change receiving antenna height. Same ERP and HAAT stations from the same distance will be close to the same strength if they are not line of sight, but if one becomes line of sight at a lower elevation, it will exceed the non LOS signal until both are LOS.

[Rich F.]

I would imagine ... an increase as the Fresnel Zone becomes unobstructed, ...

Below is a screen clip from a DOS-based program I wrote about 30 years ago that can show the effects described in the quote above, based on FCC methodology.

If the paths in the table were across free space, then the fields at the ends of the paths would be inversely related to path length, e.g., twice the path length yields 1/2 the field.

The fact that they don't in this example is related to losses from insufficient clearance of the 1st Fresnel zone for this point-point terrestrial application, which increase progressively for longer paths.

[km1125]

Been a long time since I've had to run any calculations on this, but does the Fresnel Zone affect the E-field more or less than the H-field? Does polarization make a difference?

[Rich F.]

The E and H fields in the far-field of an e-m wave always are related to each other by the impedance of free space -- about 376.73 ohms.

I'll need to research the answer to the question about polarization, or maybe someone else remembers it.

[Ben Zonia]

What programming language did you use for this program? I used BASIC, PIL, and FORTRAN over the years, but BASIC was already loaded and ready to go on both Early Apple and IBM PCs, and you can do quite a bit with BASIC, despite the disdain shown by many.

[djb]

That definitely looks like Basic (whether GW, QB, or QuickBasic). Dead giveaway is the "?" on all of the prompts for input.

INPUT commands would always prompt with a Question Mark.
LINE INPUT command would not, but only gather string (text) data; you'd have to convert the variable to numeric format to do any math.

[Rich F.]

I used QuickBasic 4.5 to write the code, then compiled it to run as an executable file for use with the DOS OS of those days.

Now to run it when I want to, I use DOSBOX running the exe on a WIN 10 PC. Kind of a pain, but I rarely use those old apps I wrote any more.

[k8jd]

Pretty much a given , reception gets better as you raise the antenna above ground clutter and noise sources. Also your horizon distance increases to enhance those distant signals more.`

[Ben Zonia]

Pretty much a given , reception gets better as you raise the antenna above ground clutter and noise sources. Also your horizon distance increases to enhance those distant signals more.`

Close to 35 years ago, I estimated that you got a 20 dB increase from a 25-30 foot AGL 10 element FM-10 with around 7 dBb gain, and I even expressed doubt that that was accurate , but it is according to the graph. I based it on a digital signal strength meter on a Technics tuner.

[Rich F.]

Below is an example plot showing the increase in field intensity vs. elevation above level Earth, for the conditions shown in the graphic. This is the maximum E-field existing between two points in space separated by a linear distance of one meter — not the voltage produced at the output terminals of a receive antenna, or at the antenna input terminals of an FM receiver.

The analysis shows that at a given distance downrange from the transmit site and except for elevations very close to the Earth, field intensity at the receive site increases nearly at a linear rate vs height AGL.

[Ben Zonia]

The Inverse Field at 10 miles would be:

138 X SQRT (6)=338 mV/m @ 1 mile inverse field

338/10 miles=33.8 mV/m inverse field

33.8 mV/m X 1000=33800 uV/m

log 10 33800 X 20=90.6 dBu

I don't understand why the graph doesn't plateau at 90.6 dBu.

Nevertheless, it seems that it would indicate plenty of room for Longley Rice 70 dBu prediction beyond 10 miles.

[Rich F.]

The Inverse Field at 10 miles would be: 138 X SQRT (6)=338 mV/m @ 1 mile inverse field. <snip> I don't understand why the graph doesn't plateau at 90.6 dBu.

The graph is showing the fields existing at a single range of 10 miles from a transmit antenna radiating 6 kW ERP from 100m AGL. That end-path field starts at the surface of the Earth and extends above it to a height of 30.5 meters AGL (100 feet).

The reason that this downrange field continues to increase rather than plateau is due to the continued reduction in Fresnel zone losses incurred for this point-point, terrestrial propagation path, as the receive height increases.

[Added later] Below is a plot of the fields that would be produced from the NEC model I wrote if it was operating in free space. Note that the 33.8 mV/m peak field shown there for ten miles is what would be expected for an inverse field of 338 mV/m at one mile — the range used in your analysis.