On 8/16/26 15:29, jonathan hunsberger via TriEmbed wrote:
I hung it up in the window and couldn't ping anything. Then i
realized that the antenna was pointing horizontally so i made it
vertical and got ping back!
This raises a detail folks should know about: polarization. Radio waves
have polarization depending on a number of factors. The two most common
flavors of polarization are horizontal and vertical. (Applications
involving space craft use other polarization types). The little rod
thing that comes with the Heltec board is expected to be kept vertical
and this gives vertical polarization (and it makes no difference if it
is pointing straight up or straight down). If two stations have
mismatched polarization the effect is attenuation of signals: typically
3-10 decibels*1 and this explains what you observed. If I monitor my
phone's wifi link to a house router I can cause the signal level to jump
up and down by changing the phone orientation.
Common antenna elements are 1/4, 1/2 and sometimes 5/8 wavelengths long.
Many antennas are built with arrays of these common size elements. The
little Heltec rod and its coax connecting to the board are around 3.3
inches (by eyeball measure) and so it's a 1/4wl monopole antenna.
Wavelength is related to frequency by wl in meters = 300/fmhz, so at the
Heltec's 905MHz operating center frequency one wavelength would be
300/905 meters or 13.28 inches and 1/4wl is 3.3.
Virtually all commercial communication equipment for stations on the
ground talking to other stations on the ground at 900mhz uses vertical
polarization. But if you have control of both stations there is nothing
to keep you from using horizontal polarization and horizontal gives you
some immunity from noise that is vertically polarized: the 3-10 decibels
attenuation is working for you in this case.
At the frequencies of interest here there is no advantage per se to
horizontal polarization: in typical use horizontal polarization is meant
to leverage the extra signal strength that comes from reflections off
the ground or water "in front of" the antenna but this effect only
generally comes into play for signals skipping off upper atmosphere
layers but 900mhz signals cannot propagate that way. So to match other
folks' expected equipment for 900mhz work you should orient single
rod-shaped antennas vertically and yagi antennas*2 with their elements
also vertical. High gain rod-shaped antennas are typically collinear
arrays of vertical dipoles. This is why they can be quite long.
Pete
*1 Decibels are tenths of a logarithmic unit. That is, one decibel is 10
to the 1/10 power, five decibels is 10 to the 5/10 power, etc. The max
power out of the Heltec board is 28 dbm. This is "28 decibels above a
reference power of one milliwatt " (the m). So that is one milliwatt
times 10 to the 2.8 power or 447 milliwatts. See this for more on the
deciblel: https://en.wikipedia.org/wiki/Decibel
<https://en.wikipedia.org/wiki/Decibel> . It should be mentioned that
the silly-small chip on the Heltec board that is an amplifier capable of
28dbm does not transmit continuously for any length of time: because it
takes a full watt of supply power to output this signal and it has no
heat sink it would unsolder itself from the board immediately if it was
somehow stuck on. The "spread spectrum" scheme for transmitting across a
range of frequencies is beyond scope (i.e. I have forgotten all I knew
about this and am too lazy to look it up).
*2 Yagi was a Japanese guy who invented the antennas we see with a
central "boom" across which is a row of perpendicular elements of
varying lengths. This type offers gain that is directional. A nine
element 900MHz yagi offers about 12dbi*3 of gain but only within a
"beam" about 35 degrees wide (where the horizontal "edges" of the beam
have 3dbi less gain than at the center and the gain off to the side and
to the rear of the antenna can be 30-40db less than at the center. So
these are useful for communication in definite directions and stink if
forced to operate with stations in arbitrary directions unless a
"rotator" can steer the antenna as needed. But there are high gain
omnidirectional, vertically polarized antennas specifically for use with
equipment like the Heltec board. These are also directional, but gain
upwards and downwards is robbed for the sake of higher gain outward
horizontally.
*3 The term dbi is short for "decibels relative to a "isotropic antenna"
reference antenna. Think of this reference antenna as representing zero
gain (in all directions). A positive number with dbi means the received
and transmitted signal are greater than what a reference antenna would
deliver and a negative number means worse. The little "chip antenna" on
an ESP32 board is an example of a negative gain number, while the little
PCB-squiggle 1/4 wavelength antenna on some ESP32 boards might offer a
small positive gain number.
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