[TowerTalk] Arrival angles and sunspot number

[n4kg@juno.com]

N4KG  comments intersperced below:

On Sun, 23 Apr 2000 15:00:13 -0600 k6ll@juno.com writes:
> 
> I don't claim to have made a comprehensive study, but just
> spent about 30 minutes with VOACAP looking at arrival angles
> with sunspots varying from 60 to 180.
> 
> It looks to me like the key question is "How far below the MUF
> am I being forced to operate?" 

	Agree.   N4KG

For example, if the MUF on a
> given path is 20.9 Mhz, contest operation will have to take
> place on 14 Mhz, well below the MUF. 

	Sidenote - My impression is that MUF's are often
	higher than most software programs predict, sometimes
	by as much as 50%.  Propagation programs seem to
	be fairly accurate at predicting WHEN the MUF will
	peak, but tend to underestimate the path MUF.  I
	assume this is partly because they are based on
	AVERAGES and we are always trying to push 
	the limits.  With a predicted MUF of 20.9 MHz
	I would feel confident that 15M would be open
	and not hesitate to try for 12M if I wanted to make 
	a sked.   N4KG


Under these conditions,
> arrival angles will be much higher. Arrival angles are generally
> lowest just at and below the MUF, and increase as the difference
> between the MUF and the operating frequency increases. 
> 
> When sunspots are high, the slope of the MUF curve is steeper,
> and the bands seem to open almost all at once. A single-op
> station just gravitates to the highest frequency band open, and
> you are soon on 10 meters. If the MUF keeps on going, like up to 50 
> Mhz,
> the arrival angles will be high even on 10. The 20 meter station at
> a multi-op, forced to stay way below the MUF, will be dealing with
> some very high arrival angles under these conditions.

	Not to mention absorption loss.  Ever notice how 10M run
	rates to Europe drop slightly around 1500Z when the sun 
	is over the middle Atlantic?  And come back up an hour
	or so later?  (This assumes an MUF > 40 MHz)  N4KG

> It's pretty easy to run VOACAP, varying only the sunspot number, and
> see what happens to the angle on a given path. I usually use
> "Isotropic+10dB" as the transmit antenna and "SW Whip" as the
> receive antenna.
> 
> In a nutshell, I don't think you can say, as a blanket generality,
> that angles are always higher during periods of high sunspots. High
> angles occur also during periods of low sunspots, such as on 20 
> meters when 15 is not open.
> 
> Dave Hachadorian, K6LL
> Yuma, AZ
> K6LL@juno.com
> 

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