HD FM transmitter - who to select from

[WNTIRadio]

2% of the audience with HD radios? I think that's being very generous. I'll bet that number is closer to .002% of the audience.

 

[Kent T]

Of every transmitter vendor, no one treats me better than Continental Electronics. Friendly, knowledgeable support, not unusual for them to support 35 year old and more transmitters very well. Nautel is the only other which comes even close.

 

[OKCRadioGuy]

Agreed. CE sets the standard even beyond Nautel and others.

 

[rfry]

... Let's assume you now have a station with 10 KW TPO. ... Using a second antenna you could use a relatively inexpensive CP antenna (e.g., ERI LP-100 or SWR 2 bay--about $3K or under), and low power feedline, say 7/8th. Then your HD exciter and a linear 250 watt power amp could drive the system.

Cheaper, perhaps, but possibly not without significant "issues."

For example, if the azimuth and elevation pattern relative fields of the antenna used for FM HD do not closely match those of the antenna used for the analog FM signal, then the liklihood increases for the digital signal to cause interference to the analog signal of that station, at some receive locations.

Please see Paper 2 at http://rfry.org for details.

 

[littlejohn]

We have found that properly mounted and designed, the selfinterference is a non-issue.

 

[rfry]

We have found that properly mounted and designed, the self interference is a non-issue.

A lot of the practical success of this approach depends on the population distribution and terrain elevations around the antenna site. If there is little population in the interference zones, then self-interference from using separate A&D transmit antennas will cause few issues (however future population distribution may change). Still, the net radiated signal from the station will not be in compliance with FCC rules in those areas.

Using a digital ERP of 10% of the analog ERP instead of 1% adds to the areas where self-interference exists.

The link below leads to an example where significant self-interference problems were related to the different elevation patterns of the analog and digital antennas.

My short, "heads up" paper on this topic which I linked to in an earlier post in this thread also was published in Radio World -- in the issue of January 17, 2001.

http://www.rwonline.com/article/the-looming-danger-of-digital-host-interference/689

 

[OKCRadioGuy]

To support what Richard is saying, here in OKC we have a higher-gain ERI Cog antenna with several FMs on it. I can take the whip antenna on my spectrum anylyzer and move it around to find plenty of positions where the digital signals actually exceed the analog. One of the broadcast groups actually shut their IBOC off due to this self-jamming.

 

[littlejohn]

I haven't found any place ours has a problem. We have a four bay half wave on a lambda section for the analog and a two bay with LH polarization just below it for hd. Works well.

 

[OKCRadioGuy]

The key might be the number of bays. The one here in OKC has a ton of elements/gain. That probably contributes to some nulls.

 

[BobOnTheJob]

Interesting observation OKC. A local station recently increased HD power and the amount of picket fencing when listening in analog beyond the 54dbu contour is enough to shave a good 10 miles off the area where even I ( a person with a high tolerance for interference ) have to stop listening. Of course, the HD coverage is dramatically better but isn't this self-interference part of the reason why -20dbc was selected in the first place?

 

[rfry]

The key might be the number of bays. The one here in OKC has a ton of elements/gain. That probably contributes to some nulls.

True, and a good observation.

Just to note that there will be far less possibility of self-interference to the analog signal caused by the digital signal in/near those nulls in the coverage area of the analog signal if the polarizations of the azimuth and elevation patterns, and the radiation centers of the co-located A&D transmit antennas essentially are equal.

The simplest way to accomplish this is to use the same antenna to radiate the A&D signals.

Collinear, interleaved A&D transmit antennas having essentially the same az-el radiation patterns and radiation centers should approximate that result.

Separate A&D transmit antennas using different numbers of bays and bay spacings, and/or with radiation centers at different elevations even on the same tower may produce wide variation in the A-D ratio at some receive sites, causing interference to the analog signal by the digital signal. Of course that can be an issue, because far more listeners now use the analog signal than the digital signal.

Using separate "c-pol" A&D transmit antennas with opposite polarization sense (left-hand vs. right-hand) probably will improve their decoupling, and thus reduce the hardware cost needed for that station to meet the spectrum mask required by the FCC for A+D FM stations.

But using such transmit antenna configurations may or may not reduce the potential of digital signal interference in the nulls of the analog signal at the receivers in those locations.

 

[WNTIRadio]

Very true with the left/right hand CP. While on the tower it can offer isolation, but after the signal bounces off a few buildings and/or hills getting to you, the purity of the L or R CP corkscrew is going to be randomized. Especially at FM frequencies. It's different for satellite, where there are no obstructions between you and the bird. And the operating frequency is much higher, where the isolation is more effective at the receive location.

One station transmits from the same 4 bay antenna at -20dBc, no problems there. Another transmits using single bay rototillers spaced 1 wavelength apart, with the opposite polarization. No problems with either one as far as interference. Of course, with single bays the nulls aren't an issue as it would be with a couple of 8 or 10 bay monsters running on the big class C's.