I understand from TVFOOL that the signal path from most of the transmitters to our location is 2Edge - Double edge diffraction. However, TVFOOL doesn't discuss how significant a consideration 2Edge is for DTV reception.

I would appreciate additional information or a link that provides more detail on this point.

Thanks

A search of this forum for Double edge diffraction turned up the following selected quotes from this original post:

(Bold added for at-a-glance reading ease.)



The LOS_h is the height where you'd start getting a line-of-sight path to the transmitter. That's the height where the path is just at the grazing edge of clearing the obstruction.

The Min_h is the height necessary to get a signal level of at least -100 dBm. Because of the way diffraction works, the signal strength at the grazing edge path is actually lower than the signal at slightly higher or lower elevations. It is actually possible to get a stronger signal below the grazing edge path than at the grazing edge path. The search algorithm to find this minimum height probably homed in on the local-maxima just above the grazing edge path to reach the -100 dBm cutoff. If this transmitter was any further away, it probably would have found no height sufficient to reach -100 dBm.

I might want to put in a column for distance to nearest terrain obstacle or angle/severity of diffraction, but I'll have to think about this some more.

Those indicators show the dominant path of the signal between the transmitter and your location. 1- and 2- Edge stand for single- or double- edge diffraction, which means that the transmission path is most like the bending of waves over one or two edges (terrain high-points, in this case). Diffraction is very common and a lot of people get strong TV signals even if they're slightly blocked by 1 or 2 ridges. Line-of-sight is not necessary for good TV reception, but diffraction can weaken signals significantly, depending on how sharp the angle of diffraction is.

Increasing antenna height for a diffracted signal can improve things significantly (depends on distance to obstruction and wavelength of signal), whereas a LOS signal will hardly improve at all with added height.

These are all great topics, and I hope my answers helped! Please keep all the good ideas and feedback coming. :)

Best regards,
Andy

Andy,

How much should one increase antenna height to minimize the negative effects of double edge diffraction? Thanks!

Andy,

How much should one increase antenna height to minimize the negative effects of double edge diffraction? Thanks!

There's no one answer to that question.

If you can take advantage of ground gain, there's an optimum height where the antenna diffracts off the ground to create a main beam that is aimed at the top of the closest hill. That height could be several feet for UHF or more normal heights for VHF.

Ground gain on VHF is discussed in depth here:
http://www.qsl.net/oz1rh/gndgain/gnd_gain_eme_2002.htm

Hi Tower Guy,

Thanks for the reply & the link.

I recently read 91% of broadcasting DTV stations are on UHF
I'd be curious to know the range of height for UHF ground gain especially if it may help to overcome the negative effects of double edge diffraction.

http://www.hdtvexpert.com/pages_c/DTVtransitionRevisited.html

Thanks for the link serndipity. Looks interesting.

I'd be curious to know the range of height for UHF ground gain especially if it may help to overcome the negative effects of double edge diffraction.

I once helped a guy in Adams, Massachusetts receive UHF signals from Albany. He used the steel deck of his swimming pool to create an artificial ground. The 4228 antenna was 3' above the deck.

You can calculate the angle to the closest knife edge and then calculate antenna height to acheive that take off angle.

Hi Tower Guy,

Thanks for the reply & the link.

I recently read
I'd be curious to know the range of height for UHF ground gain especially if it may help to overcome the negative effects of double edge diffraction.


That 91% figure was pre-transition.

It's now around 75% UHF.

How much should one increase antenna height to minimize the negative effects of double edge diffraction?

For most people in a 2Edge situation, there's not much you can do (within practical limits) to get out of it. What matters most is how sharp the angle of diffraction is when trying to reach you. For shallow angles of diffraction (just barely in the shadows), a significant amount of power can make it past the ridge. For sharper angles of diffraction (deep in the shadows), very little signal will make it to you.

The only time height makes a significant difference is when the offending obstacle is very close to you. If, for example, there is a problematic hill just a block away from you, then an increase in antenna height might be enough to reduce the diffraction angle by a few degrees. On the other hand, if there is a problematic mountain 10 miles away, it would take an enormous amount of height to even have the slightest impact on diffraction angle.

If you want to experiment with height at your location, you can try the Interactive Maps (a.k.a., TV Maps) on the tvfool web site. This tool lets you dynamically change the antenna height and see how the paths and noise margins change for each channel. You can experiment with heights up to 500 feet.

Best regards,
Andy

:)