Where can one buy a Research Communications Pre-amp

have you tried asking here?

http://www.avsforum.com/avs-vb/showt...50#post8293635

Wow.

Looking at the published specifications for the 9250, the noise figure is listed at .4 dB (I assume that is a maximum although it isn't stated as such) across the entire UHF band. That is better than what is available from the best domestic professional grade RF amplifier suppliers and therefore I would be skeptical of that figure, especially considering it would be a very difficult thing to test without highly specialized equipment.

The 127dBuV output power specification is equally impressive for an LNA. That is 40 times higher than a CM7777 (much easier to verify than noise figure). If true, it would solve alot of overload problems for those with strong local stations and weak distant ones. An amplifier with that output power for example, would not be overloaded from a single full-power station just 4 miles away, even if your 12 dBi antenna was pointed right at it!

In would be interested in knowing how they measure their overload specs. Many of the domestic ones rate overload (IM3) at, something like, "3 channels" or "5 channels".
In some areas, like SLC, you may have well over two dozen channels on the air in the UHF Band (analog full-power, analog low-power, analog translators, digital full-power, digital low-power, and even digital translators....plus, those new non-broadcast users that are starting to show up).
As you add channels, you de-rate the spec by almost 3 dB every time you double the number of stations. So, if rated at, say "+47 dBmV at 3 channels", that becomes +44 at 6 channels, +41 at 12 channels, ..........

You must assume it is a single channel ouput power specification. You only need to derate by 3 dB/channel if the additional channels are strong enough to drive the amplifier to the saturation point. Most stations will not be strong enough and each adds to the derate factor by a different amount based on there respective strengths.

Pretty typical noise figure for a GaAsFET device. Can easily be measured with an HP 8970B noise figure analyzer.

The first mixer of a receiver will overload long before the preamp will. If you have overload problems, this preamp won't help.

Ron

I disagree, it isn't typical. It's a very exceptional number for a broadband amplifier.
As an example, MITEQ inc makes some of the very lowest noise RF LNA's available today for space and military applications. Here is an example of a few of their ultra low-noise devices. Only their cryogenic devices appoach .4 dB. They run at least $1500 each.

http://www.miteq.com/micro/amps/afsa...npdf/pg42.html

The HP 8970B is a 50 Ohm instrument. The errors when measuring 75 Ohm ultra low-noise devices are significant.

While the first mixer or input RF amplifier of any particular receiver may overload, it can be prevented somewhat with attenuation between the LNA (including any line or splitter losses) and receiver. There is no downside to a high intercept point (IP3) on an amplifier other than current consumption and cost.

I installed a Research Communications preamp. It replaced an AP4700. I got a 5-10 point improvement in signal strength reading on Dish 6000 from stations at 50 miles. A snowy analog had much less snow. With the antenna pointed at two strong stations that are 10 miles away I have overload with either preamp. When pointed at stations 15 -18 miles away neither seems to overload.
John

Hey Ken, the essence of your conclusion is right, but the math isn't. Everytime that you double the number of channels, the instaneous voltage doubles, not the instaneous power. When the voltage doubles, the power goes up by a factor of four. Hence the overload resistance gets worse by 6 db whenever you double the number of stations.

The same thing happens when you increase the number of stations in a common TV or FM transmit antenna, the limiting factor becomes the peak power, not the average power, in the feedline.

See the following for the basics:
http://www.dielectric.com/broadcast/...issionLine.pdf

"Combining Channels"

"When combining channels into one transmission line both average and peak ratings must be evaluated. In general, for carriers of equal power, the peak power rises as the square of the number of carriers."

If you do the math, don't forget that all DTV carriers have a peak to average ratio of about 6 db.

For this reason, I predict that DTV reception will get much better on February 19, 2009, the day that analog stations are turned off.

It's not as exotic as you suggest. Check out this preamp kit for $35.

http://www.g0mrf.freeserve.co.uk/432LNA.htm

Point noted on 75 versus 50 Ohms (although preamps tuned for minimum noise figure are often not perfectly matched to the input impedance).

Ron

Are we not talking about a wideband RF pre-amplifier used as a receiver front end? It is receiving multiple uncorrelated rf signals at various power levels. If an amplifier is rated to deliver a maximum output of 0 dBm for example, we would not need to derate it by 6 dB just because we receive a second arbitrary channel. First, the level of that channel must be considered and second because they are uncorrelated (independent phase) they are normally added on a power basis, not voltage.
IOW, if the amplifier can deliver 0 dBm with a single input of level x, a second channel input also of level x, would mean the amplifier can only provide an output for either one of -3dBm. When the input signals do not drive the amplifier near saturation the derating factor is much less. It would be misleading to make the flat statement that doubling the number of channels means you need to derate an amplifier by any fixed amount without considering the respective levels of these signals and their degree of correlation.

Doesn't that strike you as kinda hobby shop stuff? I mean they tout better NF numbers than some major industry leaders. I'm just skeptical that's all.

The source impedance match is a good point. Low noise amps are not perfectly matched because they are so sensitive to source impedance. IOW the ultra low-noise designs may be a moot point once they are connected to a wideband UHF antenna whose impedance is anything but constant over the band.

I just ordered a Research Comms new model 9260. Does anyone have any experience with this new model. The engineer there tells me that it has a very high IP3 figure. I used the 9250 model with relatively good sucess. I live in a very polluted RF enviroment.

Isn't it wonderful that we now have HDTV preamps to go with our HDTV antennas?

Update on price:

The costs of the units are:
Type 9250/53/54 £75.00
Type 9251 £43.00
Type 9252 £20.00
Insured air mail to USA/Canada £25.00

Hokie Jim,
What is a model 9260? Doesn't show up on their website.....latest model is 9254??

I am new to HDTV so correct if im wrong isn't all tv antennas, preamps and amps HD even one that was made in the 1960's ?

Jesse,

He has a new model, the website is out of date. I have a spec sheet on it that I'll post on this board. I ordered it last Tuesday, hopefully I'll get it today.

Jim

Ok, thanks Jim...I'll be looking for it.

I still don't have the preamp. Here is the spec sheet.

 

9260 Spec.doc 25.5k . file

Research Communications is shipping the preamp tomorrow. I'll post next week and let everyone know how it works. It turns out I'm getting one of the first ones off the line.

I'm very interested because of the high IP3 figure. I've got high in band interference at my location from very strong analog and digital channels.

Hey Hokie Jim.....any update yet?

I don't have it yet.

I'll keep you posted.

Careful using these high gain, high overload type Preamps.
The output could overload your HDTV's tuner...try to keep HDTV input under about -10 dBm.
The extremely high output voltage might also trip input protect circuitry (e.g. MOVs or zener diodes).

If you have any nearby transmitters, you might need to insert additional attenuation (such as RF Splitters).

I posted an overload calculator here:
http://www.avsforum.com/avs-vb/showt...14#post9888814

PS: Whenever a manufacturer declines to stipulate whether a spec is max, min, average
or "typical" it usually means they WANT to be ambiguous....hence, I assume 0.4 dB NF is
"best case" for some (unspecified) frequency....and max might be double (triple? more?) that value...

I hooked it up last night. It is a very good preamp, but it is not the Holy Grail of reception I was looking for.

There is nothing like an antenna solution.

They quoted me $240 (amp, power supply, shipping). At 3 times the cost of any of my other preamps, I would expect 3 times the performance. I read one of the testimonials at their website from a customer that said he had gotten a "moderate but very significant improvement" over a CM 7777.

The word "moderate" threw up a warning signal in my mind. Since I have a 7777, a "moderate" improvement for $240 wasn't going to cut it.

Another poster at this forum warned me about his experience with this preamp. It and its replacement both blew up during periods of lightning.

Stacking two 4228s and two 91XGs for different situations has been my best performance enhancement regardless of preamps used.

The CM7777 has a noise figure of 2.0 in UHF, which is pretty good. You can't get a huge amount better than that; even the theoretical perfect amp can't improve SNR more than 2dB.

It's expensive, but your alternatives aren't cheap either -- an extra $60-$80 antenna plus the stacking brackets (if you can't fabricate your own), plus the extra structural loads, plus the increasing difficulty of aim. At best, stacking will give you 3dB of gain (and in practice always less, from combiner losses and the fact that you can only space optimally for one frequency). Still better than replacing a 7777 with an exotic pre-amp, but a lot more trouble and not that much less cost. Of course if multipath is the problem, the pre-amp won't help at all and stacking may help a lot.

The amount of noise that a 0.4 dB noise figure LNA produces is about 7.8 dB lower than an LNA with a noise figure of 2 dB. However at these low noise figures the anntenna noise energy will become the largest contributer of system noise in a well designed receiver system. John

The issue is not only technical but financial. At what price point is the lower noise amp a marginal return on investment? Given the fact that I haven't seen overwhelming improvement numbers for below 1 db noise amps, what exactly does the lower noise amp give one? The 10-15% improvement that I've seen quoted by users is easily wiped out by environmental conditions in one evening. Are prices such as $240 justifiable for marginal improvement?

And if one goes bad, past the warranty period, what is one's backup? Another $240...

Also an SNR of 30 doesn't give one any better picture than an SNR of 25 where HDTV is concerned.

Plus I've seen another European preamp at the same noise figure for one third the price. What gives? Which one of these is the real deal? Perhaps ResearchComms has a jacked up price because no one else was competing in the marketplace for such low-noise preamps prior to this time.

What is the real cost in design and manufacturing terms of a less than 1 db preamp? $80...$100...$150?

If you get all your desired channels 100% of the time you don't need it. I bought mine a couple of years ago and did not get the waterproof box or power inserter so now it would cost a little under $200.
My major network stations are all 50 miles away with blockage from nearby hills. FOX is the weakest and was receivable on about 60% of the evenings that I tried to look at it when I had an AP4700 preamp. When I installed the Research Comm it became receivable 90% of the time. So for me it was worth it.