HD Transmitter Questions

[Chris from Milwaukee]

Greetings all. Been reading up on HD transmission and was wondering what reject power and reflected power are all about. Also curious about space combining methods.

This HD stuff is extremely intriguing but a little over my head. Looking forward to your replies.

[NECRAT]

Hi Chris,

Glad to see ya here!

Reject power is the power returned from a device that combines RF fields. IN HD Radio you can combine the RF from two transmitters into one line. When you combine RF, most of it goes to the antenna, but by nature, a portion of it becomes rejected within the device that combines it. Reject power is essentially wasted power. You want as little of it as you can have, and you accomplish this by properly phasing the RF components into the combiner.

Reflected power is related VSWR. It is power reflected back off of the transmission system. The transmitter wants to send RF out, and it in a sense "looks" at the line. While most power goes out, items such as tuning devices, RF switches, and the length of feedline can cause a very minute amout of power to come back to the transmitter. However if a piece of the RF system fails (i.e. an antenna element gets shorted, a coax gets punctured or burns out), the impedance changes, a lot more power comes back to the transmitter. If the reflected power is too high, and is not shut down within seconds, you can suffer major damage to the transmitter (i.e. it can catch on fire).

Spacial combining is the method of using different antennas for the Main and HD signals, and using free air (the atmosphere) to "combine" the two signals. FM stations will mount an AUX antenna near their primary antenna, and feed just the AUX signal into it. There are rules for spacial combining however,
the aux antenna must be within a certain distance and height of the primary antenna, you can not use this method if your primary antenna is licensed directional, and you must license the HD antenna as an AUX antenna system with the FCC.

--Mike

[BroadcastDoc]

And if you go to NECRAT.com and look right on the front page, you'll see a multi-bay antenna with interesting spacing - the top and bottom bays are further apart than the center bays. That's actually space combining. It's a full-wavelength spaced analog antenna with a half-wavelength spaced HD antenna interleaved inside of it.

It's real interesting, and probably one-of-a-kind!

[NECRAT]

It is actually used in other sites, including another one on that very same mountain!
The antenna Chris is referring to is a Shively 6814-6R/3R interleaved antenna.
It has 6 bays full spaced, with 3 bays full spaced interleaved between the main.
It is 2 antennas mounted in one...
I've seen interleaved antennas listed, but they aren't real common, that's for sure!

Here is the FM antenna for WXXI-FM which uses a 6x2 interleaved structure by Dielectric.
It's a 6 bay full wave spaced antenna interleaved with a 2 bay full wave spaced antenna.

[BroadcastDoc]

Most of the interleaved antennas I've seen are the new dual input "panel" type...or a separate, lower antenna. But, then again, interleaved antennas weren't really readily available until relatively recently.

The inherent problems with space combining is two-fold - intermod issues, and a difference in patterns, to a point that the carrier ratios get way out of whack in certain analog nulls. There are places where the analog gets clobbered by HD and vice versa. There's typically quite a bit of modeling that goes info those antennas.

[NECRAT]

I whole heartedly agree, I'll take combining before the antenna any day now...

[tmay]

I whole heartedly agree, I'll take combining before the antenna any day now...

Have you had any problems in your analog antenna elevation nulls near the tower? I've heard some horror stories especially in dense urban areas where interleaved antennas don't match vertical nulls very well and you end up having some spots where the IBOC power ratio is so much greater than the analog, that the analog gets swamped in the receiver. Typically this only happens within about a mile or two from the tower, but can be a big problem in a large city.