Best of the ISP-Lists

Ride the Horizontal Wave

Members of the ISP-Wireless list discuss horizontal versus vertical polarity. It's yet another issue to look at when setting up a fixed wireless broadband Point of Presence (POP).

[June 29, 2001]

On the ISP-Wireless list in June, DB asked,

"For a totally brand new system in an urban area, why not go horizontal for a DSSS system? If the parts are there and not overly expensive, why don't more people do this?"

JU suggested that horizontal may actually be a great way to go:

"More people don't go horizontal for a number of reasons.

• They don't realize the potential interference reduction offered by horizontal polarization (assuming most other systems in their area are using vertical polarization).
• Or they don't believe that anyone else is out there doing what they are doing in their general area.
• Or they don't feel they can afford to spend the money to do a proper wireless survey of their area.
• Or they don't want to spend extra time and extra money locating, evaluating, purchasing, and testing horizontally polarized antennas.

In my opinion, anyone who starts deploying a new WISP system today should consider using horizontal polarization. Not learning about horizontal polarization is kind of like leaving for a camping trip with two flat tires on the car: you'll get somewhere down the road, but not as far as you expected to get."

SP, however, explained that vertical's more popular because you can go farther with it:

"Horizontal polarization has greater air loss than vertical polarization, so you get greater range from vertical polarization. This is the major reason why people started with vertical polarization. Now there's less demand for horizontal polarization antennas, so the price is higher. Now that the interference problem is becoming greater and greater, though, it's likely that more and more people will consider the horizontal polarization setup, and the price will eventually go back down."

Others didn't quite buy the air loss idea, explaining that the differences are mostly political:

[MY complained] "Where in the world did you come up with that? Vertical and horizontal radio waves propagate identically. A radio wave does not care about the location of the earth relative to it. Vertical omni antennas are more popular because they're easier to make and therefore cheaper."

[JL added] "The only reason that TV and FM started with horizontal polarization goes back to the FCC hearings of the 1950s, when the RCA electronic scan system for TV was doing battle against the CBS mechanical disk system.

Both systems were horribly sensitive to reflections (ghosts) and every effort was made to reduce ghosting. Since the trials were done in New York City, and since tall buildings tend to act as a vertical grate and pass vertically polarized signals, in order to reduce the number of signals seen by the customer, horizontal polarization was chosen.

That's basically why TV and FM use horizontal polarization, and every other terrestrial service uses vertical. When you have a forest of vertical reflectors (buildings, masts, flagpoles, vent pipes, etc), there is a greater chance of getting a vertically polarized reflection than a horizontally polarized reflection. Therefore, where multipath is an issue, horizontal works better than vertical."

—End

Related articles:
	[May 31, 2001]	Making the FCC Your Business
	[Apr. 18, 2001]	Antenna Nomenclature