Pre-amp upgrade?

[kb5udf]

Hi all,
I am currently running the following:
tower mounted Sirio 100mhz+discone->stridsberg FM filter->mini-circuits ZFL-1000VH SMA connectors with heatsink->8 port mini circuits splitter. This setup is primarily for monitoring 136mhz to 940mhz; anything is bonus and not really used much (antenna is wrong for that anyway).

Overall, I'm fairly pleased with this setup. Question is, within a reasonable price range (<500 bucks used is ok) am I leaving any performance on the table with
regard to the amp? Is there something with a much better noise figure?

I do not generally see the pre-amp causing any overloading issues and my a/b comparisons do not suggest that the current level of gain is excessive.

So the question is, can I buy an amp that performs significantly better, with similar gain (I am driving 8 receivers) and SMA connectors, or should I be content with that I have. And yes I do realize that I might get a modest bump in performance from tower mounting the amp; that may happen eventually, but not immediately.

Thanks in Advance and 73.

 

[prcguy]

The ZFL-1000 is not a good choice for a broad band preamp due mostly to the noise figure and the gain is a little high to feed a single receiver but ok for feeding an 8-way divider. For very high RF areas on a broad band antenna with no filtering I recommend a Minicircuits ZHL-1010 which is 50MHz to 1GHZ, 10dB gain, 3.5dB NF (1dB better than yours) and a whopping 46dBm IP3. For feeding an 8-way downstream the ZHL-2010 has the same specs but 20dB gain. This setup should work ok in most places but at my house its not quite high enough IP1/IP3 to survive on a Discone with no front end filtering.

For lower noise figure you have to give up high IP1/IP3 numbers and the ZX60-P103LN is worth considering but it has a gain drop off as you go higher in frequency. It covers 50MHz to 3GHz with about 22dB gain at VHF dropping to about 15dB at 1GHz and the noise figure is nice at about .5dB average. Its IP1 is 22dBm and IP3 39dBm, about 1dB higher than your existing amp. Best thing is the price at $113 or sometimes down to $50 used on eBay.

 

[kb5udf]

Thank you for the rapid and useful response prcguy. I failed to mention it but I do use some filters for bands of interest after the amp, notably for mil-air with benefit, but realize filtering before the amp is better, but I need the other bands too. In any event the post amp filtering is of benefit.

As I do not think my current preamp was being driven to much distortion (at least in bands I care about on this system), the ZX60-P103LN+ looks like the ticket.
Two final questions:
1. What kind of power connector is that? (my current amp was pre-wired)
2. Will any old nominal 5v switching wal-wart function well? If not what should I get?

Thanks again

 

[prcguy]

The ZX60-P103LN+ has solder connections for power. I would use a linear power supply and not a switcher, that might put RFI right into the preamp.

 

[kb5udf]

Got it. Thanks again.

 

[arg0s]

For very high RF areas on a broad band antenna with no filtering I recommend a Minicircuits ZHL-1010 which is 50MHz to 1GHZ, 10dB gain, 3.5dB NF (1dB better than yours) and a whopping 46dBm IP3.

You have to be careful when there is 10dB less gain. No matter how good the NF of the preamp is, the noise figure of the whole system will be at best equal and at worst higher than the present setup. The OIP3 of the preamp should be decent but is normally secondary because the receivers are the limiting factors and determine in this regard the 2-Tone dynamic range respectively the IIP3 of the system.

 

[Ubbe]

I am currently running the following:
tower mounted Sirio 100mhz+discone
This setup is primarily for monitoring 136mhz to 940mhz

Can you please exlain the antenna setup. Is it a 100MHz Sirio antenna and a discone and they are somehow connected together? I was thinking that the first thing to do are to install a FM broadcast filter, but if you monitor FM broadcast then it can't be used.

Discone antennas have some 10dB-15dB loss in the 700-900MHz band due to its directivity are no longer pointing at the horizon. Just by using another antenna for the 700MHz-900MHz band would increase the signal as much, or more, than any amplifier. You will have to get a diplexer that has one antenna connector for signals above 600MHz for the vertical 700MHz-900MHz antenna and another connector for below 600MHz for the discone.

When working with amplifiers you should always have a variable attenuator to adjust the signal level to see if a lower signal level can improve the noise/signal ratio and also check how much signal you can have without a receiver overloading when running a cable from the amp directly to a receiver. When a receiver starts to overload its sensitivity gets worse and the signal disappears more and more until it goes into full intermod where you hear all sorts of signals on frequencies they do not actually transmit on.

/Ubbe

 

[kb5udf]

arg0s I'm planning to go with the 20db version of ZX 60, since as best as I can tell, the current preamp is not causing overloading issues.

Ubbe the antenna is the Sirio SD2000N. Signal strength for 700-900mhz is not a primary concern for this particular antenna system and for the local systems I monitor signal strength in this range is not a problem. Also, I am not using the amp/preamp to improve weak signals or to overcome coax losses. I am using it to feed an 8 port splitter. Additionally, as I think I previously noted, I am using an FM broadcast filter, before the amp/preamp and 8 port splitter. A few of the ports coming out of the divider have coaxial dynamics 225-400mhz filters for my military/government listening, and are beneficial for my SDR's. I do have other antenna's more suited to 700+ for when I need them, including a tower mounted 900mhz commercial antenna and an attic mounted, homebrew high 800mhz vertical. At my location though, the limiting factor for 700-900 reception is rather severe broad banded cell system noise and I have at least one of my supplemental antenna systems with some inline filtering in this regard.

thanks to all who responced

​

 

[iMONITOR]

Hi all,
I am currently running the following:
tower mounted Sirio 100mhz+discone->stridsberg FM filter->mini-circuits ZFL-1000VH SMA connectors with heatsink

Why would a heat sink be necessary on a pre-amp? I never heard of them getting hot.

 

[prcguy]

Why would a heat sink be necessary on a pre-amp? I never heard of them getting hot.

Larger transistors biased to handle high RF levels = more current, more heat.

 

[iMONITOR]

Larger transistors biased to handle high RF levels = more current, more heat.

Would these be present in a receiver preamplifier? I've never had any preamps get warm, let alone hot. I normally don't even see them with heat sinks.

 

[prcguy]

Would these be present in a receiver preamplifier? I've never had any preamps get warm, let alone hot. I normally don't even see them with heat sinks.

I have a bunch of preamps that have heatsinks. Pretty normal for the high level crowd.

 

[iMONITOR]

I have a bunch of preamps that have heatsinks. Pretty normal for the high level crowd.

Interesting. Can you post some pics and explain when/why they would be used? I thought all "receive" preamps were low power/low level.

 

[prcguy]

Interesting. Can you post some pics and explain when/why they would be used? I thought all "receive" preamps were low power/low level.

There is low level as in too low that will make reception worse due to IMD produced inside the preamp vs adequate signal handling specs where the preamp is mostly transparent except for increasing levels. Take for example the piece of junk AR2 receive only preamps with a 1dB compression point of 0dBm or their better models at 12dBm. These run off 10-16v and draw about 25ma of current or about .3 watts. Without a preselector filter they will almost always create lots of IMD raising the noise floor and producing "IMD ghost signals" throughout the band.

Then there is the ZX60-P103 series mentioned above with a 1dB compression point around 22dBm (10dB higher than the AR2) and runs off 5v at about 100ma or .5 watts of power. This preamp will survive a lot better in high RF areas. Then look at my suggestion of a ZHL-1010 with a 1dB compression point of 26dBm running off 12v at 525ma or 6.3 watts. This preamp will work in even much higher RF areas without being pushed into IMD land.

The problem is, except for the old AR2 types that are way out of date, the higher level preamps will have a higher noise figure and that's just something you have to live with when dealing with higher RF levels.

 

[iMONITOR]

There is low level as in too low that will make reception worse due to IMD produced inside the preamp vs adequate signal handling specs where the preamp is mostly transparent except for increasing levels. Take for example the piece of junk AR2 receive only preamps with a 1dB compression point of 0dBm or their better models at 12dBm. These run off 10-16v and draw about 25ma of current or about .3 watts. Without a preselector filter they will almost always create lots of IMD raising the noise floor and producing "IMD ghost signals" throughout the band.

Then there is the ZX60-P103 series mentioned above with a 1dB compression point around 22dBm (10dB higher than the AR2) and runs off 5v at about 100ma or .5 watts of power. This preamp will survive a lot better in high RF areas. Then look at my suggestion of a ZHL-1010 with a 1dB compression point of 26dBm running off 12v at 525ma or 6.3 watts. This preamp will work in even much higher RF areas without being pushed into IMD land.

The problem is, except for the old AR2 types that are way out of date, the higher level preamps will have a higher noise figure and that's just something you have to live with when dealing with higher RF levels.

All of that sounds like very low power. Where does the need for a heat sink come in?

 

[prcguy]

All of that sounds like very low power. Where does the need for a heat sink come in?

Heat sinking becomes necessary when the preamp case is inadequate to dissipate the power consumed. Less than 1 watt is usually not a problem and no heatsink should be needed. In the case of the ZHL-1010 consuming 6W of power its necessary.

 

[kb5udf]

The ZFL1000VH I have is quite warm to the touch even with the heatsink; it is clearly needed. This gives me fond memories of my early ham days, rag-chewing on a 5 watt HT with a friend a mile or two away. The radio (Alinco DJ-580t) had a metal case, at least on the rear, and it would become like handling a hot potato (painful to the touch) after a long chat session.

 

[arg0s]

I'm planning to go with the 20db version of ZX 60, since as best as I can tell, the current preamp is not causing overloading issues.

Discone antennas aren't known for high output signals. I guess there is also a considerable lenght of coax cable involved between antenna and preamp? If the present setup doesn't overload, the new amp shouldn't give you trouble either. The optimum for such broadband operations would be a push-pull amplifier but I don't think Mini-Curcuits has one with such a low noise figure.

What do you expect to achieve by using a new amplifier that your present amp isn't able to give you? Receiving more (DX) stations, better signal-to-noise ratio?

All of that sounds like very low power. Where does the need for a heat sink come in?

The received signals have a huge dynamic and range from sub uV to the V range. To be able to amplify not only the small but also the high/very high signals with the same linearity, the amplifiers need to be operated in class-A with high DC currents and therefore very low efficiency (<<10%). As most of the DC power is converted to heat, the need for a more or less sizeable heat sink comes in.

 

[Ubbe]

The guys that do complex measurements, at radio sites that have huge RF levels, had a preamp to their measurement instruments that got so hot that you couldn't touch it and they had to periodically let it cool down. A scanner had a small plastic transistor for its mixer and it had overload issues. Then came version two and the transistor had been replaced by a bigger metal can type and overload issues where gone. The more current thru a transistor the less risk of overload but more molecules are moving (or is it just the electrons pushing each other in a chain reaction), its those that create friction heat, and will then also produce more noise. To reduce noise you try and make the molecules move slower, by cooling them down to near absolute zero for radio telescopes. So it usually impossible to have both low noise performance and high overload capability.

/Ubbe

 

[kb5udf]

arg0s wrote: "What do you expect to achieve by using a new amplifier that your present amp isn't able to give you? Receiving more (DX) stations, better signal-to-noise ratio?"

Answer: from my reading possibly about 3db better signal.

This antenna system is dedicated to what hams would typically call "weak signal" work, such as distant military aircraft
some of whom are doing training missions in the Gulf of Mexico, south of the Florida panhandle. Also, when the
atmospheric conditions are good,, I can receive the Joint National Capital Region, army trunking system from
Ft. Polk, which I believe is about 90mi away. I realize 3db isn't much, but my decode ratio on unitrunker is sometimes
right on the ragged edge of receiving Ft. Polk, and that trivial improvement might be just enough for a substantial
improvement in ability to actually decode the p25 audio.

Also, I have a secondary antenna system in the attic dedicated to local "strong signals", and always wanting to do better, I plan to replace the CATV amp/splitter currently in use there with the ZFL and another mini circuits splitter. That secondary/attic system is dedicated to monitoring local trunking systems and/or repeaters, so generally, if I have a little less signal from the attic system its not a big problem. The attic system
also gives me a way to continue monitoring a good deal if I have to unplug my primary tower system due to LIGHTNING (an annoyingly frequent problem in these parts).

I would add that I am quite pleased with my tower mounted Sirio discone. I have found their products well built
for a consumer model and the little one on my tower seems to be doing quite well for Mil-Air and reasonably
good for Civil air.