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

[Eric Gustafson]

Just one small nit to pick Bill.

It is Eglin AFB not Elgin AFB.  I used to live about 2KM off the west end of the SAc runway there.

73,  Eric  N7CL


-----Original Message-----
>From: "W7EXG (Bill)" <W7EXG at q.com>
>Sent: Jan 30, 2019 10:10 PM
>To: 'Southern AZ DX Association E-Mail Reflector' <sadxa at mailman.qth.net>
>Subject: [SADXA] Elevation Angle as it relates to FT8  - de W7EXG
>
>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
>
>
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