Friday, May 21, 2010

Unusual antenna proposed in Toronto

CHHA-1610 Toronto has proposed a novel (for broadcasters) new directional antenna.

The station has been operating at 10,000 watts daytime/1,000 watts nighttime, non-directonal, using a 22.5m Valcom loaded whip antenna in downtown Toronto. Due to unexpectedly good DX conditions (excessive skywave..) CJWI Montreal, on the same frequency, has suffered interference. Industry Canada asked the station to temporarily reduce nighttime power to 250 watts - which addressed the interference but left CHHA unable to reach much of their northwest Toronto audience.

CHHA proposes a directional antenna, with a null protecting CJWI. They'll operate at a power of 6,250 watts day and night, using the directional antenna at all hours with the same pattern. At a traditional AM station, this would be accomplished by erecting two towers.

CHHA proposes to do it with only one tower -- and a "Hot Guy Wire".

The single tower will be roughly 50m high. The guy wire facing due east will be insulated from the tower, at the top, by an insulator similar to that used on high-voltage AC power lines. Another insulator will "break up" the guy wire, electrically, about 25% down from the top of the guy wire. A third insulator, an 8-inch ceramic "egg", will be used at the base of the wire. A 4.1uH inductor will be installed between the bottom of the guy wire and an earth ground.

The "floating" guy wire will serve as a second, reflecting element to the antenna system. It will "deflect" much of the power that would otherwise be radiated towards Montreal, and direct it instead to the north and south. (right into the Toronto neighborhood the station wishes to reach)


Doug Smith said...

Oops, I forgot the link to the application. This is a .ZIP file that will open into eight documents submitted by CHHA.

Bernd Waniewski said...

Interesting story! Telefunken (Germany), where I worked in the antenna department, realized an antenna in Langenberg Rommel (Germany) where we applied the same idea: At Rommel we used 5 ropes of a cage on one side of the mast and one rope of the same cage on the other side of the mast for achieving the directivity. Hence the distance between the two radiators became greater which acts against the small bandwidth of such an antenna. By the way we offered a similar design to Radio Vatican years ago. But it was never realized.
I am surprised that it seems to be possible to obtain such a high directivity by just tuning the "hot wire" by an inductance. At Rommel both radiators are fed, which eases the tuning of the system. Nevertheless it took me a long time to achieve the wanted pattern. I assume that the bandwidth of the antenna in Toronto must be narrow, which leads to some effort in the antenna tuning elements to compenmsate it. Fortunately the power is low.
I wonder how the vertical pattern looks like.

Bernd Waniewski

Doug Smith said...

(I think
this link will make your reference clickable)

It looks like your antenna delivers a similar directional pattern to the Toronto installation. The partially-split guy wire arrangement (on the lower set of wires) is essentially unheardof in America.

Most directional antennas in North America are MUCH more directional -- have a much deeper null. (you may have heard of cases where listeners can see the transmitting antenna but cannot receive the station! - although usually interference is the problem, not weak signals)

I would suppose the high frequency of the station would mean the narrow bandwidth wouldn't be a serious problem. (at least not compared to if the same antenna design were tried at, say, 630KHz)

Doug Smith said...

Today (August 19th) the CRTC denied this application.

The reasons were non-technical. The station had filed its "annual returns" (I'm not sure what those are...) for 2006, 2007, and 2008 three months late. CRTC policy is to deny changes requested by stations that are not in compliance with the regulations.

This seems a bit strange, given that the station's returns for 2008 and 2009 were apparently filed on time.