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post #1 of 31 Old 06-13-2010, 05:34 PM - Thread Starter
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The lower noise factor of the RC preamps is appealing.
Per TV Fool FAQ:
Quote:


Be aware that amps and pre-amps will actually cause you to lower your Noise Margin. No matter how much gain an amp or pre-amp claims, it will actually reduce your Noise Margin by the amount listed as the Noise Figure (NF) in its specs.

However, there have been reports in the forums of RC preamps needing to be returned for repair.

Per RC's website:
Quote:


In order to obtain the very low noise figures offered we use PHEMT transistors. These are more sensitive than Mosfets to pulse transients caused by storms. To help combat this we added low frequency input filtering to the preamps which has proven to be very effective. Since we started using it less than 2.5% of our customers have had to return units for repair.

An important consideration is that the antenna should be properly grounded to a copper spike. This gives the transients a direct route to earth and reduces the likelihood of damage by at least 50%.

It's sensible to use a surge arrestor on the mains input, as you would to protect any equipment in the home.

I'm wondering how prevalent this issue may be among AVS members:
  1. Which RC model preamp do you own?
  2. How old is it?
  3. Has it ever required service? If yes, how many times?
Hopefully, the newer serial # units are less likely to have this issue.

Thanks for sharing your RC experience.

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post #2 of 31 Old 06-14-2010, 06:35 AM
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Quote:
Originally Posted by ota.dt.man View Post

The lower noise factor of the RC preamps is appealing.

Have not used the RC preamp yet. Am looking into it and the new Kitztech.
http://www.kitztech.com/

I disagree with Andy at Tvfool that preamp will degrade noise figure. A good preamp will have much lower NF then typical TV. Reports I've see are that NF for TVs is around 7 dB. Assuming preamp has enough gain so signal arriving at TV exceeds that level NF in TV can be ignored. In that case system NF is determined by preamp.

Quote:


From Ken Nist's site:
Normally the signal to noise ratio will be set by the receiver's first transistor. But if an external amplifier is added, the first transistor in that amplifier determines the S/N ratio. (Since the external amp will greatly magnify its own noise as well as the signal, the receiver's noise becomes insignificant.) Since there is no reason to think the external amp's first transistor is quieter than the receiver's first transistor, there is generally no benefit to the S/N ratio from an external amplifier.

Receiver Noise
Actually there is a reason to think the external amplifier is quieter than the receiver. Long ago designers made an effort to make the TV's first amplifier stage very quiet. But now 90% of homes use cable or satellite boxes (strong sources) and most of the rest are rural homes using antennas that have mast-mounted amplifiers. So the TV's noise is rarely a factor. Some TV makers no longer put any effort into making their sets quiet.

http://www.hdtvprimer.com/ANTENNAS/basics.html

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post #3 of 31 Old 06-14-2010, 09:55 PM
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Quote:


I disagree with Andy at Tvfool that preamp will degrade noise figure. A good preamp will have much lower NF then typical TV. Reports I've see are that NF for TVs is around 7 dB. Assuming preamp has enough gain so signal arriving at TV exceeds that level NF in TV can be ignored. In that case system NF is determined by preamp.

I do agree that the Noise Figure of tuners built into TV sets are generally not that good. Most TV sets will get around 6 dB while a few have been known to be as bad as 9 or 10 dB.

However, the point of the discussion (taken a little bit out of context here) is that the antenna is the only component in an RF chain than ADDs to your Noise Margin (makes the value increase). All other components (including pre-amps) will cause some amount of Noise Margin loss.

Good amps will have a minimal impact (reducing Noise Margin by only around 1 to 3 dB). This may actually hide the influence of bigger losses that occur down the line (e.g., cable loss, splitters, and receiver NF), thus resulting in an overall improvement in net Noise Margin when you consider the entire chain. As Tschmidt pointed out, the overall end-to-end net Noise Margin will be dominated by the antenna's gain (ADDed) and the pre-amp's Noise Figure (SUBTRACTed).

I just want to make sure people avoid the mistake of ADDING an amps gain specs to the Noise Margin spec thinking that they have a 30 dB gain antenna because that's what the label on the box says (usually a crappy antenna mated with a cheap high-gain amp). That would be wrong and hopefully people will avoid making this mistake.

Best regards,
Andy
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post #4 of 31 Old 06-16-2010, 01:43 PM
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9254 here...two years old...never had any problems....it is a GREAT preamp for certain situations but is more prone to overload than some.

Eastern NC.
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post #5 of 31 Old 06-16-2010, 08:25 PM - Thread Starter
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Quote:
Originally Posted by Jesse31 View Post

9254 here...two years old...never had any problems....it is a GREAT preamp for certain situations but is more prone to overload than some.

Hi Jesse,

Glad to hear your RC-9254 has been reliable.
How common are lightning induced surges in your area?
To which preamps are you comparing it for potential to overload?

Thanks for the report

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post #6 of 31 Old 06-17-2010, 04:46 PM
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Well the OP knows what I have but I will restate it anyway.

I have the RC 9262 attached to my antenna system at 70ft on the tower. Works fine for me and I'm pretty happy so far.

Ryan, N2RJ

Opinions expressed are solely my own, and not that of my employers, its parent company, affiliates and subsidiaries.
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post #7 of 31 Old 06-17-2010, 05:31 PM
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Here are a few spectrum plots directly after the power inserter.

WABC is VHF 7, pretty strong. To the left of it is non-TV spectrum to give you an idea of the noise level. To the right of it is WNJB-DT
KYW-DT is UHF 26, marginal, off the side of the antenna (philadelphia). To the left is WASA-LD, and to the right is WTBY-DT
WNJJ-LD is UHF 41, pretty strong. So strong I can receive it with an indoor antenna here. But the Research Comms amp seems to handle it well, not overloading.
LL
LL
LL

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post #8 of 31 Old 06-17-2010, 05:38 PM
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Here is W36AZ analog (NJN PBS LP translator) as well.
LL

Ryan, N2RJ

Opinions expressed are solely my own, and not that of my employers, its parent company, affiliates and subsidiaries.
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post #9 of 31 Old 06-17-2010, 05:38 PM
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I've been watching this for some time.
http://www.avsforum.com/avs-vb/showthread.php?t=773819
Anyone compare old to new or are they lasting?

"The purpose of diplomacy is to prolong a crisis." Spock, Mark of Gideon, TOS
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post #10 of 31 Old 06-17-2010, 10:36 PM - Thread Starter
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Quote:
Originally Posted by raj2001 View Post

Well the OP knows what I have but I will restate it anyway.

Hi Ryan,

Thanks for posting the spectrum plots.

Between Ryan's very positive review of his new RC 9262 LNA and the TV Fool FAQ quote in the first post of this thread, I think the RC line of preamps are appealing. The 9262 also filters out UHF above Ch 51.

Hopefully, AVS member reports in this thread over time will show the newer serial # units have better surge protection than the older models.

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post #11 of 31 Old 06-18-2010, 07:59 AM
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Quote:
Originally Posted by ota.dt.man View Post

Hi Jesse,

Glad to hear your RC-9254 has been reliable.
How common are lightning induced surges in your area?
To which preamps are you comparing it for potential to overload?

Thanks for the report

Hi,
I was using a CM0064 and also tried a Winegard 8780...neither overloaded...I thought!...the 9254 overloaded quite a bit as I lost some channels...finally found that all preamps were overloading some (15 miles from two strong stations)...I eventually went to a winegard HDP-269 and used the 9254 as a distribution amp...the 9254 is a very good amp if overload is not a problem.

Lightening is quite prevalent around here.

Eastern NC.
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post #12 of 31 Old 06-19-2010, 08:05 PM
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Quote:
Originally Posted by Jesse31 View Post

Hi,
I was using a CM0064 and also tried a Winegard 8780...neither overloaded...I thought!...the 9254 overloaded quite a bit as I lost some channels...finally found that all preamps were overloading some (15 miles from two strong stations)...I eventually went to a winegard HDP-269 and used the 9254 as a distribution amp...the 9254 is a very good amp if overload is not a problem.

Lightening is quite prevalent around here.



The RC amps aren't likely overloading at least in-band. Probably you are experiencing intermod from lack of front-end filtering. Try either a HLSJ or a UVSJ at the input. OTOH, they most definitely ARE prone to blowing from nearby surges or lightning strikes. I've blown three or four. I no longer use them as they just aren't rugged enough for outside installation. I do however continue to use one as a post-amplifier inside the house.
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post #13 of 31 Old 06-20-2010, 04:59 AM - Thread Starter
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Quote:
Originally Posted by cpcat View Post

OTOH, they most definitely ARE prone to blowing from nearby surges or lightning strikes. I've blown three or four. I no longer use them as they just aren't rugged enough for outside installation.

Per Research Communications:
Quote:


In order to obtain the very low noise figures offered we use PHEMT transistors. These are more sensitive than Mosfets to pulse transients caused by storms. To help combat this we added low frequency input filtering to the preamps which has proven to be very effective.

Hi cpcat,
  • How long ago did you purchase these units? - Are they a previous generation, before the low frequency input filtering was added?
  • About how long did each one last before a surge damaged them?
  • What preamp(s) do you now use for outside applications?

Thanks

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post #14 of 31 Old 06-20-2010, 07:19 AM
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Quote:
Originally Posted by ota.dt.man View Post

Per Research Communications:

Hi cpcat,
  • How long ago did you purchase these units? - Are they a previous generation, before the low frequency input filtering was added?
  • About how long did each one last before a surge damaged them?
  • What preamp(s) do you now use for outside applications?

Thanks

I've used both the older models and the newer ones. I've also sent the blown units back for repair to RC more than once and had them add the LF filters. I've also used Nextek lightning arrestors on both sides of the amp and they still blew. They seem to be extremely sensitive to even air-borne static surge. Maybe a heavier metal enclosure would help. You could maybe also add metal foil to act as shielding inside the weatherproof case.
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post #15 of 31 Old 06-20-2010, 07:37 AM - Thread Starter
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That's disappointing to hear especially since they have such a low NF.
So what do you now use for outside applications?

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post #16 of 31 Old 06-20-2010, 01:05 PM
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Sitco PA24 for broadband UHF and a CM7777 for hi vhf.
The Sitco is much more rugged and performance is very close to that of the RC amps.
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post #17 of 31 Old 06-25-2010, 05:04 PM
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Quote:
Originally Posted by ota.dt.man View Post

The lower noise factor of the RC preamps is appealing.

However, there have been reports in the forums of RC preamps needing to be returned for repair.

I'm wondering how prevalent this issue may be among AVS members:
  1. Which RC model preamp do you own?
  2. How old is it?
  3. Has it ever required service? If yes, how many times?
Hopefully, the newer serial # units are less likely to have this issue.

Thanks for sharing your RC experience.

I have the RC9245
Purchased in Dec 2008.
Yes it was serviced.

Details of the service: I think it stopped working in Sep/Oct 2009. However my family and I were getting ready to move so I took down the antenna tower etc so we could move in Dec.

Because it was winter when we moved, and I did not have super motivation to install antennas until this spring, I never did get it repaired until last month.

If I recall, it was $12 to ship and took a couple days to get there. Their techs had it fixed within 1 business day and back in the mail to me. It took twice as long for it to show back up in my mailbox. I guess the cheapest UK international shipping is done in a Vespa or something.

I will be erecting the antenna tower (CM1630) and horizontal stack of 91XG's with 5x boom sections and my Funke PSP.1922 in the next week or 2.

I have to run over to my friends business and grab about 300' of 7/8" (875 series) 75 ohm coaxial line for the optimal location.

Out of warranty repair was £12. Not bad.

To the best of my knowledge it was not a lightning strike that damaged it. I really have no idea. The feedline had a grounding at the top just after the pre-amp. I had a grounding strap on the cable shield at the base of the tower. I also used a 25 pair phone line fused grounding block on the rotator control. I also had appropriate grounding at the entry to my house.

The symptons were:
"The PSU has a light when the switch is on. However, when the LNA and
PSU are removed from the line, there is almost no difference in signal
readings with my meter. I have short cabled the LNA, PSU, and ATSC tv
right at the base of my antenna and there is very little difference
between no LNA and LNA."

Their response was:
"It sounds as though the preamp transistor has gone down. Do you have a way to measure current drawn? If the LNA is taking either no current or trying to take more than 75 mA then the transistor is not working correctly and we'd need to replace it. Providing the psu is delivering 8V then that is fine and there would be no need to return that."

Since mine no longer has a warranty, and it as been repaired, if it happens again, I will crack open the LNA and see if I can get the transistor from mouser or digikey. Much faster repair time and I have a surface mount rework station if it is a PCB inside.

One final note:
Their customer service is outstanding. I'm not sure if Ellen is one of their tech's, or if the tech's give her all the info she needs. But you can ask them any question about the device and she will provide you with a very detailed answer. They are fast to respond via email. Even though the pre-amps may be fragile, I will continue to use it.

Try asking ChannelMaster for additional info about intermod and overload.

~ryan
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post #18 of 31 Old 06-25-2010, 05:12 PM
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Quote:
Originally Posted by Jesse31 View Post

9254 here...two years old...never had any problems....it is a GREAT preamp for certain situations but is more prone to overload than some.

Here are the overload specs from an email I had with RC:

"Blocking, intermodulation or harmonics caused by very strong local signals does not occur in the 9254 until the preamp output exceeds +18 dBm (2 volts) / 127 dBuV.

The CM7777 overloads at -17 dBm (40 mV), a 34 dB lower level.

In short, the 9254 will handle very much stronger signals without trouble.

If in the future you find you have a specific problem with one of the stations then an input filter would help. Unfortunately all filters have a loss which adds directly to the noise figure of a system, so really they are best avoided if possible."


This includes all signals that your antenna and pre-amp receive. so if you had an antenna (or multiple with HLSJ and UVSJ's) you would have from 50-800Mhz. This includes FM (look at FM fool and see if you need an FM filter), the entire 700-800 Mhz band now, and if you have a very active VHF/UHF Ham operator nearby.

~ryan
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post #19 of 31 Old 06-25-2010, 07:30 PM
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Quote:
Originally Posted by b1gmoose View Post

Here are the overload specs from an email I had with RC:

"Blocking, intermodulation or harmonics caused by very strong local signals does not occur in the 9254 until the preamp output exceeds +18 dBm (2 volts) / 127 dBuV.

The CM7777 overloads at -17 dBm (40 mV), a 34 dB lower level.

In short, the 9254 will handle very much stronger signals without trouble.

I find it troubling that an RC employee who is qualified to discuss the subject would express his estimate of overload levels in such imprecise terms.
Quote:


...If in the future you find you have a specific problem with one of the stations then an input filter would help. Unfortunately all filters have a loss which adds directly to the noise figure of a system, so really they are best avoided if possible."...

Another near useless answer by BP. Cheap band reject filters (HL/SJ. UV/SJ) typically attenuate the pass frequencies by about a dB. Cheap FM reject filters that reject the entire FM band or the portion from 93 MHz and up but are now hard to find, attenuate the pass band signals by one to two dB, whereas the commercial grade, cylindrical ones can often attenuate the pass band signals by two or three dB.

The insertion loss of UHF bandpass filters varies tremendously and unfortunately, the pricier ones tend to attenuate more than do the cheap ones. There is usually less insertion loss introduced by $40 Channel Master Jointennas than by $200 Blonder Tongue tuned filters. Also, the insertion losses I have measured in identical Blonder Tongue BPFs and long discontinued Tru Spec BPF UHFs actually vary by a few dB from one unit to another.

Remarkably, the insertion loss of Blonder Tongue's deepest channel 2-6 filters can run as high as ten dB.

I haven't measured insertion losses on the few UHF notch filters I inventory, but I'll have the opportunity to measure some Winegard UT-2700s and a couple of Microwave filter priducts tomorrow and will report the passband loss figures tomorrow night.

Unfortunately for the do-it-yourselfer, it is very difficult, and sometimes impossible, to self tune notch filters and to evaluate their unintended attenuation. Blonder Tongue has discontinued its premium notch filter, the MWT-u. Tin Lee will not only tune filters to your exact specifications, they will even "tweak" the tuning to your situation if asked. I have to BPF some relatively weak channel 38 signals in the presence of a strong channel 36, and Tin Lee has nudged the center of the passband to a little above channel 38 for me so as to maximize the attenuation of channel 36.
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post #20 of 31 Old 06-26-2010, 05:14 AM
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There's nothing wrong with RC and I'd agree their customer service is good. The problem is that PHEMT preamps are delicate and thus inherently prone to failure. If you are willing to replace/repair it every year or two then go for it.
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post #21 of 31 Old 06-28-2010, 02:51 AM
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Quote:
Originally Posted by b1gmoose View Post

Here are the overload specs from an email I had with RC:

"Blocking, intermodulation or harmonics caused by very strong local signals does not occur in the 9254 until the preamp output exceeds +18 dBm (2 volts) / 127 dBuV.

The CM7777 overloads at -17 dBm (40 mV), a 34 dB lower level.

In short, the 9254 will handle very much stronger signals without trouble.

~ryan

Quote:
Originally Posted by AntAltMike View Post

I find it troubling that an RC employee who is qualified to discuss the subject would express his estimate of overload levels in such imprecise terms.

Yes, it would be good to know exactly what these numbers are. They sound like 1db gain compression numbers.

http://zone.ni.com/devzone/cda/tut/p/id/2952

The +18 dBm value is pretty typical of GaAsFet transistors used in the RC9254, while -17 dBm is typical for a regular lo-noise bipolar transistor. If the -17 dBm figure is to be believed, it would suggest that the design of the CM7777 is ancient and hasn't been changed since the 70's.

The +18 dBm figure for the RC9254 also means that this preamp is much more capable of overloading the receiver it's connected to. It can deliver 64 milliwatts to the receiver, while the CM7777 can only deliver 0.02 milliwatts.

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post #22 of 31 Old 06-28-2010, 09:41 AM
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Quote:
Originally Posted by cpcat View Post

The RC amps aren't likely overloading at least in-band. Probably you are experiencing intermod from lack of front-end filtering. Try either a HLSJ or a UVSJ at the input. OTOH, they most definitely ARE prone to blowing from nearby surges or lightning strikes. I've blown three or four. I no longer use them as they just aren't rugged enough for outside installation. I do however continue to use one as a post-amplifier inside the house.

That was with the front end filtering you mentioned...was overloading.

Eastern NC.
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post #23 of 31 Old 06-28-2010, 05:56 PM
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I've had two RC amps in the past and both went down with nearby lightning strikes.For the last couple of years I've had an Emcee PA-20U-75 low noise preamp with no problems.Seems to work just as well as the RC amp,but lightning hasn't blew the magic smoke out..I'm no RF whiz,but I assume the specs are good.Here's a sheet on it.

http://www.emceecom.com/Documents/EM...e%20062207.pdf

http://www.emceecom.com/Documents/EM...-Amplifier.pdf
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post #24 of 31 Old 06-28-2010, 07:01 PM
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Wow, pretty cool.
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post #25 of 31 Old 06-28-2010, 10:16 PM
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Quote:
Originally Posted by MAX HD View Post

I've had two RC amps in the past and both went down with nearby lightning strikes.For the last couple of years I've had an Emcee PA-20U-75 low noise preamp with no problems.Seems to work just as well as the RC amp,but lightning hasn't blew the magic smoke out..I'm no RF whiz,but I assume the specs are good.Here's a sheet on it.

http://www.emceecom.com/Documents/EM...e%20062207.pdf

http://www.emceecom.com/Documents/EM...-Amplifier.pdf

How much $?

Ron

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post #26 of 31 Old 06-29-2010, 06:43 AM - Thread Starter
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A Google search turned up the following HDF post
Quote:
Originally Posted by Rick0725 View Post

Here is a broadcast quality UHF preamp, 20 DB gain and .5DB noise temp. Model PA-20U-75 $499.

http://www.tonercable.com/Product.aspx?ID=1615


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post #27 of 31 Old 06-29-2010, 09:20 AM
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Originally Posted by ota.dt.man View Post

A Google search turned up the following HDF post

And don't forget the S&H
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post #28 of 31 Old 06-29-2010, 09:37 AM
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I would think that very few people live in an area "quiet" enough to reap the rewards of a preamp with a .5dB NF.
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post #29 of 31 Old 06-30-2010, 07:50 AM
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Originally Posted by TV Trey View Post

I would think that very few people live in an area "quiet" enough to reap the rewards of a preamp with a .5dB NF.

Maybe not but it certainly helped me.

Ryan, N2RJ

Opinions expressed are solely my own, and not that of my employers, its parent company, affiliates and subsidiaries.
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post #30 of 31 Old 06-30-2010, 08:38 AM
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Quote:
Originally Posted by raj2001 View Post

Here are a few spectrum plots directly after the power inserter.

Ryan, what spectrum analyzer did you use to provide the plots shown here?
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