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0.8 MHZ spacing interference question

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bmw
Posts: 1027
Joined: Sun Dec 05, 2004 1:02 am

0.8 MHZ spacing interference question

Post by bmw » Mon Apr 08, 2019 9:00 pm

When the 106.9 translator for 1480 WIOS went on the air last year in Tawas City, it all but obliterated 107.7 WHSB out of Alpena.

However, today, 106.1 Tawas went off the air for a short time but 106.9 was still on, and WHSB was fine.

In other words, when either 106.1 OR 106.9 are on the air but not both, then 107.7 comes in fine. It is only when both stations are on that 107.7 gets toasted.

What is the explanation for this?



CK-722
Posts: 412
Joined: Sun Dec 16, 2018 3:53 pm

Re: 0.8 MHZ spacing interference question

Post by CK-722 » Mon Apr 08, 2019 10:04 pm

RITOIE. Two strong signals mix and produce RITOIE at 2A-B and 2B-A, where A and B are the frequencies of the strong signals. If the third station's signal is weak, it wil be captured by the RITOIE product. It has to do with the harmonic of one signal mixing with the other signal. The cheaper and more poorly made the radio, the worse the problem. It can be all but eliminated with proper design, expensive as it may be.

Example:

A=106.1
B=106.9

2A-B=2(106.1)-106.9=212.2-106.9=105.3
2B-A=2(106.9)-106.1=213.8-106.1=107.7

A 250 watt translator close by can have a very strong signal, as strong as a more powerful one further away. Are you near the translator antenna?


Is THAT where they got the idea for the 486-SX?

Same (x, y, z), different (t)

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bmw
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Joined: Sun Dec 05, 2004 1:02 am

Re: 0.8 MHZ spacing interference question

Post by bmw » Tue Apr 09, 2019 12:12 am

Thanks for the explanation - I figured it has something to do with harmonics.

The "obliteration zone" is quite large. On US-23 in Tawas City, the translator is about 6 miles straight-line away. It is completely unlistenable in town and it isn't until almost halfway to Oscoda before the interference mostly clears up - probably as far as 12 miles away from the translator. And I have a quite expensive Blaupunkt car receiver that has otherwise excellent selectivity and overload rejection.



CK-722
Posts: 412
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Re: 0.8 MHZ spacing interference question

Post by CK-722 » Tue Apr 09, 2019 7:16 am

Since it's that strong from that far away, something else is probably going on. Since 106.1 and 106.9 are collocated and close together in frequency, the harmonics and mixing may be happening in one of the transmitters. They should have their engineer check that out. Just a few watts radiated could do that. I doubt RITOIE could happen except close to the translator, within a mile or so of the transmitter. I was thinking it was on the WIOS tower.

I remember riding through Tawas City when I was quite young, listening to our transistor radio and trying to find the WIOS tower. The WIOS signal was strong as you went past the communications tower along US-23, and not knowing then where the towers were, thinking that might be the tower. When I found out a few years later they were directional, I knew that wasn't it. Probably, if I had known where to look, I would have seen them.

The Carrolls have built quite the broadcasting empire since selling WKMF to Beck Ross Communications, of Long Island Fame. I remember when Dick Bing did the signoff cart on WKMF, and always hearing that it was owned and operated by "The Carroll Broadcasting Company". Then one week Dick said "Beck Ross Communications". I said, to myself, who is Beck Ross Communications? Hard to believe it's been over 50 years. I was a radio geek teenager then. Now I'm a radio geek Senior Citizen. Turns out there is quite a History of both licensees.


Is THAT where they got the idea for the 486-SX?

Same (x, y, z), different (t)

Your bullet missed my trial balloon.

RTN Price. Not guaranteed. As of 12:30, 157.71 Down 0.22.

Artificial Intelligence is a Child that needs a Parent to guide it through.

innate-in-you
Posts: 205
Joined: Wed Oct 31, 2018 8:54 am

Re: 0.8 MHZ spacing interference question

Post by innate-in-you » Tue Apr 09, 2019 11:39 pm

I wonder how the math would explain intermod I experienced in Mt. Clemens a few years back.

We were at a picnic, and one guest was bound and determined to listen to the Tigers play on 97.1. There was a cheap AM/FM/2SW band radio, but our guest just couldn't get acceptable reception.

I suggested he listen to 1270 AM instead, but he said he refused to listen to the game on AM. (the AM was still simulcasting the games then).

I picked up the radio, played with the band switch, the volume control, and tuning control for more than a minute, and adjusted the antenna in such a way that I set the length by marking two thirds of my finger at the end (doing this entirely for show!), then placed the radio in front of him and said don't touch it!

He then listened to the game to its end, not noticing it was actually tuned to 1270.

Now, this is only about two miles from a Class B FM DA with near-maximum power on 102.7 - but no other FM anywhere near at the time.
I never found the mathematical key to figure out the cause of the RITOIE.



CK-722
Posts: 412
Joined: Sun Dec 16, 2018 3:53 pm

Re: 0.8 MHZ spacing interference question

Post by CK-722 » Wed Apr 10, 2019 10:24 am

Could be blanketing interference close to "WBRB-FM" by any other name, and there have been plenty! Cheap radios usually don't have good front ends, so it lends itself to more blanketing interference. I wonder what a SuperRadio 2 would have worked like in that situation.


Is THAT where they got the idea for the 486-SX?

Same (x, y, z), different (t)

Your bullet missed my trial balloon.

RTN Price. Not guaranteed. As of 12:30, 157.71 Down 0.22.

Artificial Intelligence is a Child that needs a Parent to guide it through.

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