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Minimum signal strength to feed a translator

J

joebtsflk1

I've been doing some informal consulting for a gentleman who wants to buy a translator that has to be fed over the air. Problem is though the predicted receive signal for the parent at the translator is under 30 dBu.

So, how low can you go and receive a decent mono signal? At under 50 dBu I'd think atmospherics would affect the signal occasionally, but that's something he'd have to live with. In Des Moines there's a translator that used to receive its parent over a 100 mile path. The parent, KUNI Cedar Falls was over 500 meters HAAT w/ 100 kW, but the predicted signal was only in the 35 dBu range at Des Moines.
 
There are many factors, beyond the raw signal strength that affect reception. First, plot the azimuth of the primary station from the translator. Look for co-channel and first adjacent stations on or near the path. If you find any co-channel stations within 100 miles or first adjacents within 35 miles, you could have trouble. Next, decide how high up your receive antenna is going to be. Remember that the predicted contours are based on an antenna at 30 meters. 30dBu may be more like 40 or 50 at 150 feet. The other thing to remember is that the predicted field strength is only a model. The FCC model predicts that the field strength will exceed the specified level at about 50% of the measured locations. In practice, received strengths vary by 20 to 30dB just by moving a few feet, even vertically up or down on the tower. 30dBu is actually quite a lot of RF. It sounds bad on your car or home receiver because the actual strength near the ground is often 20-30db less than at 10 meters up.

Have you listened to the primary station's signal on a car radio at the translator site? If it's there and reasonably solid, your chances are good. I've seen class A stations rebroadcast at over 50 miles and class C stations at more than 150 miles. Use a very good receiver and a big yagi, cut to the exact frequency, if possible. There are several excellent receiver choices, including one from BW Broadcast.
 
I have one in Northern Massachusetts that works in full quieting stereo with 48dBu. The trick, so to speak, is using the BW Broadcast RBRX-1. After putting that in, I wouldn't use anything else.

The primary is a non-comm class B 14kW @ 610' 82 miles away. Receive site is a Scala log periodic, 260' up the tower. If there's a full power station on your tower or nearby, be sure to use a notch filter.

The nice thing about the BW receiver is that you can literally tweak every parameter, from filter bandwidth to HF filtering and more. It then reconstructs a clean composite output, complete with composite clipper to catch any of the peaks generated in the process. Also does RDS as well.
 
I should have mentioned that the predicted signal strength in my first post of >30 dBu was with the receive antenna at 100 feet. (I've been using Canada's CRTC Longley-Rice map program) The proposed parent station could be received in the area of the translator on a car radio earlier in the year, but it's sketchy right now with summer heat and humidity. Azimuth between receiver and parent is clear on the co-channel, but he's got first adjacents on either side, but neither of them are locals.

But perhaps bringing the receive antenna up, say another 50 feet, could help. We might not have the luxury of going up 260' on the receive site, though.

The proposed parent station is 25 kW at 328', and will be about 85 miles away from the translator.

Have heard about the BW receivers beyond very selective. Do you use stacked Scalas or is it a single?
 
Depending on the direction of any co-channel interference, you may find a Scala Log-Periodic antenna is a better choice than one of their Yagi's. If co-channel interference is a possibility from the rear the receive antenna, the Log-Periodic will do a better job of rejecting it. Otherwise, a Yagi will probably be a better choice. Scala makes both and they are excellent. After a lot of trial and error, I've settled on using them along with the BW receiver. So far, I've never needed to stack the antennas. It is the best combination I've come up with and I wouldn't consider anything less. I've wasted a lot of money experimenting with cheaper components. There is no reason for you to do the same.
 
If you're having issues with a weak signal, try using a phased array of 2 log periodic antennas. Receiving a mono 3.5 watt translator at another site 28 miles away using them.

The log periodic won't have problems with side lobes like the Yagi will, if you have either co-channel or adjacent channel stations blasting away off to the sides of what you want to pick up.
 
3.5 watts at 28 miles, that's quite a hop...sounds like the BW receiver w/Scala log periodic is a good combination. Appreciate all the suggestions.
 
Shively now makes a log periodic too. Haven't used one yet, I'm sure they're solid. Just experience from the Scala antennas is that they're built like tanks. Never had one fall apart on me, even 300' up on a tower that takes a good deal of ice.

The 3.5 watt translator is in mono, which helps quite a bit. Was using the Sony HD tuner prior with decent results, but nothing touches the BW receiver. It ain't cheap, at almost $2k, but it's worth it. It also has web access, which is great for monitoring. The Sony had to be hot rodded with an outboard power supply/battery/float charger combo because it loses it's mind when power is removed and returns to an "off" state. Not good in the dead of winter on a remote mountain top.
 
Doubt that turning off the stereo pilot on the parent station will be real popular with the owner. But, hopefully there might still be enough signal to allow the translator to receive a decent mono signal, though it would be better if the parent could be mono too.
 
You should also test the channel for RF when the primary is off the air some evening. You don't want your translator suddently rebroadcasting another distant station if the primary goes down or if the weather is unfavorable. You might consider using RDS or another subcarrier on the main station to key the translator.
 
joebtsflk1 said:
Doubt that turning off the stereo pilot on the parent station will be real popular with the owner. But, hopefully there might still be enough signal to allow the translator to receive a decent mono signal, though it would be better if the parent could be mono too.
Click to expand...

Is it really necessary to turn off the pilot at the transmitter if the receiver can be forced to mono? (I suppose that depends on what the receiver means by "mono" -- ignore the L-R subcarrier, or just sum the decoded L & R?)
 
In some areas, tropo is common in the summer, which will pose a problem with the translator translating another station. Or in the case of an HD2 translator, the HD reception would drop out and the translator would broadcast dead air or the HD1 of the primary. There's a pair of HD2 translators near me that succumb to tropo knocking out HD reception. One will broadcast dead air, and the other will broadcast the HD1. It depends on the HD radio that is used to feed the translators. But that's a moot point, because if there's tropo, the translators' frequencies will have a strong co-channel.
30 dBu is definitely too weak for HD even with an APS-13 antenna. Depending on the strength and location of the primary station, tropo might either enhance its signal or bring in a different station.
I wonder if it would be possible to put a receiver closer to the primary station, and use a Marti to get the audio to the translator site.
Or you could just translate the primary station's Internet stream, like I've heard some translators do. That may be against the rules if it's commercial.
 
If this hasn't been mentioned, if your receive frequency is say 90.5 and there's an IBOC station on 90.3 or 90.7 (even at 50 miles or more), it will tear your 30dbu signal a new one. And even if there isn't, a future IBOC activation can take something that worked fine and kill it dead--and there's nothing you can do about it. Consider this carefully before spending money.
 
If it's a noncomm translator in the reserved band, then just feed it directly and don't try to fish a weak signal out of the mire. The ones I've done, some are noncomm primary stations but the translators are in the commercial band requiring an off air pickup.

The reserved band ones I have, I use either a Barix if it has to be done on the cheap or a Tieline BridgeIT if there's a bit more of a budget. The nice thing about the Tieline is that there is return audio which is great for monitoring.
 
"The trick, so to speak, is using the BW Broadcast RBRX-1" YES. The RBRX-1 is your best weapon receiver-wise, if you can afford it. The Sony is nearly as good, but has to be modified. I have both, and have compared them for DX purposes. I was involved with a project where I had two groups I was involved with that had issues with each other's FX allocations. One was listening to their primary class A 50 miles away, looking right through the other groups new translator, 400 ft up, running 250 watts, only four miles away ADJACENT CHANNEL! Guess what worked? Yup. I tested with my RBRX1 and it worked, pulling in the I.F. to 62k. There was still enough left for stereo and RDS even. The original translator that was getting receiver interference suddenly didn't have a problem anymore. We ordered another one, and cured the problem. Since then, I've had an experience where a co-located aux site for a station tore up our translator input. Guess what cured that problem? Yup. The RBRX1 stopped the problem cold. The new fancy Crown Broadcast/ Chrisso module DID NOT. Another little thing to know comparing the Chrisso internal module to the BW Broadcast RBRX1 is that the BW will repeat RDS. The 800 dollar Chrisso module does not. In some cases the Chrisso module for Crown translators is a nice option, but be careful on what you use them on. They aren't quite a RBRX1.
 
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