IBOC Question/Problem

[Jim in Western Vermont]

I am hoping someone can fill me in on whats up with this.

The only station with IBOC right near me is 103.9, about 12 miles away.

About 2-3 times a year their IBOC hiss is present throughout the entire band
when aimed in their general direction. This can last several weeks & then
it will be fixed. I understand that the IBOC would affect 103.5, 103.7, 104.1 & 104.3 with 103.7 & 104.1 being the most affected.

Aimed off from them it does disappear. It also is gone about 20 miles
up the road.

When I report the problem they tell me "We will look into it" but nothing gets done for weeks.

I work near Burlington & 107.9 has IBOC & I have never had this problem with them.

Does anyone know whats happening here & I would also be curious
if anybody has had a similiar problem.

Am I correct or wrong to assume:

The station on 103.9 has no right to interfere on frequencies that far from their own.

Upon being notified of the problem they should either fix it if they can or shut the IBOC off until they can solve it.

Any suggestions on what I can do are appreciated. As a DX'er I am getting tired of this recurring problem from this same station. :banghead:

[Mark]

There are several transmitter configs to do IBOC, the best way is so-called low-level combining in a fully solid-state transmitter. This is the cleanest because the solid state amplifier has the least amount of distortion, thus the least out of channel garbage.

Then there is the same low-level combining into a broad-banded tube amplifier, the performance and amount of out-of-channel garbage varies dramatically with the tube amplifier’s tuning.

Then there is high-level combining and mid-level (rare) combining wherein two separate transmitters are used, most often the station’s existing tube-type transmitter (for analog) along with a new solid-state transmitter (for IBOC). The outputs are combined and fed to the antenna and also in this case the tuning and interaction between the transmitters can cause problems.

Then there are stations that use separate antennas, if the antennas are not closely matched there can be problems on-channel when you are in areas where ratio of the digital to analog gets too high.

The better IBOC exciters (first stage of system) actually analyze the transmitter system’s output and pre-correct the signal to compensate for distortions in the combining and amplifier stages. Some early IBOC exciters did not have this feature.

Next time you call you might inquire what type of IBOC system they use.

[w9wi]

I guess I'd think the separate-antenna system would be the best as far as keeping things linear and avoiding the spreading Jim is reporting. Also the most energy-efficient.

That said, as you suggest it's also the most likely to result in self-interference, when the patterns of the two antennas don't match exactly and in some places the digital to analog ratio is way too high.

Jim, it really shouldn't even splatter into second-adjacents. (103.5 and 104.3 should be clean) I have no trouble whatsoever DXing second adjacents to local IBOC stations. Obviously first-adjacents are another story.

[Jim in Western Vermont]

Thanks guys for getting back...

For the most part the 2'nd adj. are ok...but not clean as they should be.

I guess I have no option except to talk with an engineer at the station &
see what they will do.

Today, it's not thru the band but centered around 103.9
In other words it starts at about 101.9 & goes thru 105.9
with a sharp cutoff. (101.7 & 106.1 is totally normal)

Still not the way it should be but better than the entire band. Now whats strange is as soon as I hit the point where it goes away (via moving the antenna)...it's gone totally & even 103.9 & 104.1 are better. Normally those would be trash unless I phase it out (not easy but I've done it) so something changed even from last night.

I don't know a lot of the technical aspect of this but I have learned a few things from reading your responses. Much appreciated !

[Jeff-MA]

99.5 WCRB Lowell, MA also has this problem, and it's been there for over 2 years. When you're within about 2 miles or so of the transmitter, their IBOC hash can be heard at least from 98.3 to 100.5, probably even wider than that. This is the only station I've ever noticed with this problem. Here at home, 43 miles away, I don't notice it, it's just on 99.3 and 99.7, where it should be.

[Jim in Western Vermont]

Hi Jeff,

I should mention that when the problem was going on
that it would block semi's out on frequencies as far
away as 101.9....this condition lasted up to about 20 miles
up the road (driving away from the station) as I checked it out one morning going to work.

I am about 20 miles from the transmitter but it's a clear shot
so that could explain why I hear IBOC on 103.5 & 104.3 when aimed
there however aimed away I can DX those frequencies.

The problem with their IBOC gone wild has cleared up for the most part.

Jim in West Vermont
Yamaha T-80 Mods
Andy Bolin Phaser
APS-13 / McKay Dymek DA5 AM antenna
20+ yrs in radio / 35+ yrs DX'ing