[SADXA] Elevation Angle as it relates to FT8 - de W7EXG

[W7EXG (Bill)]

SUBJECT: Elevation Angle as it relates to FT8 
From: Bill W7EXG

During my presentation on 40m DX DAY Propagation Corridor, I made a point
regarding that a 40m Dipole/Yagi at 0.55 wavelength height, or a Vertical or
a 4 Sq. antenna, the elevation angle is around 23 degrees (+/-).  This
elevation angle is simply the angle at which there is a maximum of RF energy
density from your antenna.  Of course, there is much radiation above and
below this 23 degree angle. 

I would like to point out that the greatly reduced RF energy at greatly
reduced lower elevation angles is likely most often being used by FT8 mode,
and not by CW or SSB modes.  I am sure most of you are aware that for a
fixed antenna height, the power radiated density degrades with lower
elevation angle, and most of you are aware that low elevation angles yield
higher MUF and longer DX. 

But I think the table below, makes an interesting point you may not have
thought of regarding FT8, when these two effects are combined. 

Anyone with an Antenna Simulation program and access to the Digital Ionogram
Data base (online free) can generate the following illustrative tables
below.

DAYTIME 

Currently (1/30/19) the Elgin AFB (30.4 deg Lat) Data base is showing peak
foF2 (Critical Frequency for the F2 layer) of about 5.3 MHz at their solar
noon.  

Assume, as reference, ANY signal with an elevation angle of 27 deg into the
F2 region, running 100W at that time and space. 

Below is a table of signal strength vs elevation angle vs MUF = M (foF2).
[The M factor was discussed for E layer, but a similar graphic can be
generated for M factor of the F2 layer]. 

DAY TABLE
ELE ANG(deg)      REF(dB)   PWR(W)  MUF(MHz)

27			0 dB	  100		10.4
13			-3	   50		16.1
8.5			-6	   25		18.5
5			-10	   10		20.3
1.5 			-20	    1		21.4

What this table shows, is station running 100W on a 15m antenna (1/2 wave
height) (21.075 MHz) will be acting as a QRP rig, running instead 1 Watt on
15m (21.075 MHz). The higher Angle 15m RF power with lower MUF will be going
into space. Your 15m KW is radiating 10W of reflected power on 15m. 

Today, (1/30) I listened to 15m FT8 frequency 21075 kHz about our solar noon
period, and heard many weak FT8 signals. (I did not hear can CW or SSB
signals) I must also point out that I did NOT hear any 12m FT8 signal 24916
kHz at that time or the remainder of the day.  Thus I am confident the MUF
limit (for 1.5 deg angle of radiation) was between these frequencies. 

NIGHT
Currently (1/30/19) the Elgin AFB (30.4 deg Lat) Data base is showing foF2
(Critical Frequency for the F2 layer) of about 3.2 MHz at around 2am local
Elgin time. 

Assume as reference ANY signal, with an elevation angle of 27 deg into the
F2 region, running 100W. 

NIGHT TABLE
ELE ANG(deg)      REF(dB)   PWR(W)  MUF(MHz)

27			0 dB	  100		6.1
13			-3	   50		8.9
8.5			-6	   25		10.0
5			-10	   10		10.7
1.5 			-20	    1		11.19

What this table shows, is station running 100W on a 30m antenna (1/2 wave
height) (10.136 MHz) will be acting as a QRP rig, running instead from 1 to
10 Watts on 30m. The higher Angle 30m RF power with lower MUF will be going
into space. 

I recently listened to 30m FT8 frequency 10136 kHz and could heard weak FT8
signals throughout the night.  I must also point out that I did NOT hear any
20m signal 14076 kHz at that time or the remainder of the night. 

SO WHAT DOES THIS ALL MEAN?

These Tables clearly show:
A)	Low angle RF has perhaps > 20dB attenuation from the main lobe, and
is more like a QRP signal. (To get 1W of reflected power at 1.5 deg, you
need 100W)
B)	This low RF power with low angle of radiation has a much higher MUF.
C)	This results in very weak signals at much higher frequencies. When
you tune and find the highest frequency FT8 signals, they are generally
weak. They are likely coming effectively from a 1W to low wattages sources. 
D)	If you call CQ on CW with 100W under these same conditions, your
effective 1W CW signal might be heard, but not copy-able, or ignored. 
E)	The main lobe RF had a lower MUF, and went out into space. Most of
the lobe also whet to space, since having a lower MUF.  In some cases, only
about 1% of the power was actually reflected at 1.5 deg.   
F)	FT8 Technology is a weak signal decoder based on low bit rate
transmissions. It is well suited for receiving weak signals emanating from
1.5 deg elevation radiation at the highest possible MUF. 

LESSONS LEARNED: If you hear a weak FT8 SIGNAL on an otherwise dead Higher
Frequency band, FIRE UP YOUR LEGAL LIMIT LINEAR AMP AND CALL CQ (CW/SSB) ON
THAT BAND. YOU MIGHT ONLY BE REFLECTING 15 WATTS, BUT hey, SOMEBODY WILL
HEAR YOU and MIGHT RESPOND. You might need to ask them to QRO!! 

73's
Bill W7EXG