To preamp, or not to preamp...

[kd8mkg]

This building and this little patch of earth are the only things keeping me from receiving my local tower and ground frequencies clearly. I can hear them, but they're way down in the noise. It only opens the squelch occasionally, so even the carrier is weak.

Most of my experience is as a ham, as such, I see preamplification as a tool to be used with caution. In the ham world, my goal is to lower the noise floor as much as possible. I want my S-meter reading close to zero when no signal is present. I use my HF radio's attenuation way more frequently than I use it's preamplification. But this is in regards to HF. In the VHF/UHF world, all of the popular local repeaters are on absurdly tall towers, I can get into them with full quieting using a 1 watt handheld in my basement. As a result, I know very little about VHF/UHF preamplification. Logically and unfortunately, height is almost an enemy for airport transmitters. They want Signals aiming slightly upward toward the horizon, with coverage only in their area of concern... they don't care about ground coverage.

Would this situation benefit from a preamp after the filter, perhaps with a variable attenuator? If so, what preamp might you suggest? (Bonus points if it uses N connectors.)

Note: This line accounts for both the height of the airport's transmit antennas and my receive antenna.

 

[Ubbe]

You don't mention what antenna you are using and where it is installed or what receiver type you have.

I've been to several remote sites for air traffic controllers and looked at the local radio antennas at airports, and they mostly use standard dipole or GP antennas. They don't need to direct their signal upwards as you can use any type of antenna for that kind of range. It is when aircrafts are far away that you'll need that coverage and that will be close to the horizon.

I've used a PGA103+ based amplifier at work where I where next to Stockholms tallest building with at least 25 channels at work, and I couldn't detect any problem when having the amplifier before the filter. The amplifier never overloaded, it was scanners who did that and needed an extra filter. It's absolutely critical to adjust the signal level using a variable attenuator to get the exact level where you get the best signal/noise in a receiver. You can try that attenuator both before and after the amplifier to test if it overloads and creating additional noise in the signal and also try any filters both before and after the amplifier.

A normal scanner has something like 5-6dB internal noise and a PGA103+ has 1dB and amplifiers have always helped me improve reception even when having the amplifier at the receiver end of the coax and not at the antenna. But it have never helped when using a professional 2-way radio, they use a too good receiver design. Even a cheap $100 TYT-MD380 radio couldn't be improved using an amplifier.

But even with a 4-5dB improvement of an AM signal, that sometimes doesn't open squelch, will still be extremely noisy. But experimenting with amplifiers and attenuators and filters are always a good exersise to improve your knowledge and add some experience.

I believe that MiniCircuits have a PGA103+ amplifier with N connectors. But variable attenuators mostly comes with F connectors and are 75 ohm. There are much cheaper PGA103+ amplifiers on Ebay that use SMA connectors that works just as good, there's nothing different between the ones you can get, except that some can be powered from the coax, a good thing to have if you install the amplifier at the antenna.

Whenever I use a shortwave receiver I have to attenuate the signals due to all the interference, I can never use its preamp. HF are very different to VHF/UHF.

/Ubbe

 

[MDScanFan]

I will echo Ubbe asked...what antenna are you using? Also, what is the height of the antenna?

It is good news that you can at least detect a faint signal. That means there is hope. I was in a similar situation dealing with an obstruction to an airport ground station and signals were detectable but very faint.

For my situation the order of impact (most to least) was the height of the antenna, the antenna, and amplification/filtering. For my tests I played with a handheld, a half-wave dipole, and several feet of coax. Raising the antenna from the ground floor to 20' helped the most. I swapped a yagi for a dipole and that improved things as well. From there, adding amplification helped further. When I tried to simply add amplification/filtering to the haldheld at the ground floor I saw negligible benefit.

In the end I wound up with an elevated yagi with a preamp/filter (to minimize cable loss required to put the antenna where I want it). Signal strength is much improved. Good luck.

 

[jwt873]

For FM, an amp does very little. But for weak signal SSB/CW work, they can be beneficial. For really weak signal work (like moonbounce), a preamp is absolutely necessary.

Years ago I bought an eBay preamp just to see if it would do anything. Can't recall exactly what I paid, but it was cheap.. in the 5 - 10 dollar range. (I just searched eBay now and found this similar unit).

2PCS 0.1-2000MHz RF Wideband Amplifier 30dB Low-noise LNA Broadband Receiver | eBay

Amplifier gain：. -This product can be applied to various rf receive front-end and increases communication distance. Amplifier working frequency of the lower limit is subject to input and output capacitor, the default value is 0.1 uF, working to 0.1 MHz.

www.ebay.com

I tested it with the Kenwood TS-2000 I was using back then. The amp really did make a difference. In the video below, the radio was receiving a UHF CW beacon at 432.300 MHz. The beacon is 60 miles away over flat prairie. When the vid starts, you can just hear it. At the 15 second mark, the amp is switched in. You can see that it did make a difference.

I never got around to using the amp. It lived in a drawer for quite a few years. Out of interest... I dug it out recently and tried it again. This time it wouldn't amplify anything. I'm thinking I might have zapped it with static electricity one day when I touched it.

 

[kd8mkg]

You don't mention what antenna you are using and where it is installed or what receiver type you have.

I will echo Ubbe asked...what antenna are you using? Also, what is the height of the antenna?

Currently, I'm using a BCT15X. My antenna is an Arrow 144/440 J-pole mounted in the attic of my single-story ranch house. So it's not ideal. The SWR around the civilian air band is still relatively low, though, around 2-3:1. The top of the primary element is touching my roofline at about 15.5' AGL. This is a temporary antenna. I have a DPD MilTenna arriving next Tuesday, I expect that to help some. I'm also using a Mini Circuits ZBSF-95+ FM broadcast notch filter.

I'm quite limited in terms of antenna placement. My house is a 1-story in front, and grades down to a 1.5 story in the rear. I am near the top of a large hill, which does help some. In the rear, there is a deck. There is a power pole about 30' behind the deck, and there are overhead lines feeding my house electricity.

The one antenna improvement I could make is to get it outside onto a mast affixed to the 4x4" deck post furthest from the power lines. Unfortunately, due to the power lines, the antenna could be no higher than about 17' above the surrounding ground level. Any higher would risk contact with the power lines in the event of mast failure.

Edit: Note that my house is about 55 years old, and my roof is about 20 years old. It's fairly unlikely that there is a metal foil radiant barrier.

My house has a hipped roof with gutter on all four sides, which makes roof placement not impossible, but challenging. Anything I do needs to be temporary, as I plan on eventually erecting a 50' tower on the North side, away from the power lines. All of these problems will vanish when I am able to do that, but that could be several years away.

But it have never helped when using a professional 2-way radio, they use a too good receiver design. Even a cheap $100 TYT-MD380 radio couldn't be improved using an amplifier.

That's interesting, and a bit confusing. In addition to the scanner, I have 5 other transceivers and 2 other receivers. Even with my compromised antennas, I can easily receive anything of interest in the surrounding area with these radios. I can clearly hear one 2 meter repeater in particular that's about 75 miles away with my Anytone, it's about 58 miles past the airport in the same general direction.

 

[Ubbe]

At the 15 second mark, the amp is switched in.

It have happened a lot in development of RF amplifiers recent years. Those high gain 30dB amplifiers often have bad overload performance and intermodulation. I hope you used a 20dB attenuator, or if the receiver had one internal, or you would probably have added a lot of extra noise from overloading the front end.

/Ubbe

 

[jwt873]

This was just a raw test to see if a $10 eBay pre-amp would do anything. I never planned to, or did put it into service.

Apart from the gain in dB, there is also a noise figure which needs to be considered. If I were really serious about a 432 pre-amp.. I'd go for something like this: BEKO HPP-432 (430-450 MHz) Mast-Mount Preamplifier - Island Amplifier BEKO RF-KIT USA

 

[kd8mkg]

I hope you used a 20dB attenuator, or if the receiver had one internal, or you would probably have added a lot of extra noise from overloading the front end.

/Ubbe

This practice seem strange to me. Instead of using too much amplification, and then an attenuator, you'd think it would be favorable to just use an appropriate level of amplification. Instead of +30/-20dB, why not just have a +10dB preamp? Or a variable gain architecture?

I can understand something like +30dB before a really long coax run, and then a variable attenuator near the receiver, but amplifying too much and then attenuating seems like unnecessary redundancy in most residential use cases.

If this is common practice, I'm sure there is valid reasoning, I just don't really understand it.

 

[MDScanFan]

How long is your cable run and what are you using?

Maybe just wait until you get the MilTenna before making any other changes. It should work a bit better than the J-pole but I would suspect it would not be a night and day difference if mounted in the same location. Once you have it you could try a test in the attic and then a temporary mount outside to see if there is much of a difference. You could use a handheld radio and a short jumper for the testing.

One thing to consider is when you do not have line of sight to the tower then you are relying on diffraction off of the objects in between. That type of propagation means there will likely be locations that work poor and some that work better on your property. When you are outside perhaps you could try moving the antenna around to see if you can pinpoint a spot where it works better. Move it several feet laterally or vertically. Or try a different spot altogether.

Currently, I'm using a BCT15X. My antenna is an Arrow 144/440 J-pole mounted in the attic of my single-story ranch house. So it's not ideal. The SWR around the civilian air band is still relatively low, though, around 2-3:1. The top of the primary element is touching my roofline at about 15.5' AGL. This is a temporary antenna. I have a DPD MilTenna arriving next Tuesday, I expect that to help some. I'm also using a Mini Circuits ZBSF-95+ FM broadcast notch filter.

The one antenna improvement I could make is to get it outside onto a mast affixed to the 4x4" deck post furthest from the power lines. Unfortunately, due to the power lines, the antenna could be no higher than about 17' above the surrounding ground level. Any higher would risk contact with the power lines in the event of mast failure.

 

[Ubbe]

If this is common practice, I'm sure there is valid reasoning, I just don't really understand it.

30dB are 1000 times amplification and 20dB are 100 times. It's usually easier to make a good low noise amplifier that can handle large signals if not focusing in producing maximum gain. But we still want a reasonable amount of gain.

When electrons move and encounter resistance they produce heat and noise. If I exaggerate and say that a coax has a 1uV level of noise when it transports electrons and we receive a 2uV signal from the antenna. Then at the receiver we have 1uV noise added to a 2uV signal and that's 50% noise. If we amplify the signal to 20uV from the antenna before it enters the coax then we still have 1uV of noise but that's only 5%. The receiver might not be able to handle that big signal without creating noise of its own, so we attenuate down to a 2uV signal at the receiver and at the same time the coax noise are also attenuated down to 0,1uV, still 5% noise level.

Coax has attenuation of its own and the signal might be reduced to half at the other end of the coax but its noise are still 1uV and the noise percentage would double. Without amplification we would end up with as much noise as signal, and if the signal where even less it would drown and disappear in the coax noise and can never be recovered again.

So we will always try and use as high signal as possible until we reach the receiver. Ultimately the attenuation should be in the receiver just before the first active component, the mixer or 1:st amplifier stage.

/Ubbe

 

[dlwtrunked]

This was just a raw test to see if a $10 eBay pre-amp would do anything. I never planned to, or did put it into service.

Apart from the gain in dB, there is also a noise figure which needs to be considered. If I were really serious about a 432 pre-amp.. I'd go for something like this: BEKO HPP-432 (430-450 MHz) Mast-Mount Preamplifier - Island Amplifier BEKO RF-KIT USA

Actually NF (noise figure) is the first thing to consider, not the second. Do not buy anything over 2 dB. Then enough gain to make that pay off. If a NF is not published, do not even think of trying it as it is likely not good. 30dB gain is indeed to much for your use. A low NF and say may be 20 dB gain would be good but a filter in going to be needed in many cases. Expect to pay $100 or more for a good pre-amp. finally, the 3d IP needs to be considered in regard to the pre-amp generating modulation. Pre-amps bought just because of their high gain will lead the buyer (as some here do) to wrongly believe pre-amps do not help weak signals.

 

[kd8mkg]

How long is your cable run and what are you using?

It's about 50' of LMR-400. The coax definitely isn't the issue.

One thing to consider is when you do not have line of sight to the tower

Technically, I have direct line of site to the control tower. The control tower at the local airport is brand new and 265' tall. There are a number of antennas on top of it, but there is also another cluster of antennas at another location on the airport property which are at lower height. With a sufficiently powerful telescope, I'd literally be able to see the top of the tower from my deck, I suspect the Tower and Ground frequencies aren't coming from the tower itself, I suspect they're are being transmitted from that other antenna cluster (although it does resemble an RCAG site).

 

[MDScanFan]

Understood. My comment was referring to wherever the transmitter antenna is located. The tower or otherwise.

Technically, I have direct line of site to the control tower. The control tower at the local airport is brand new and 265' tall. There are a number of antennas on top of it, but there is also another cluster of antennas at another location on the airport property which are at lower height. With a sufficiently powerful telescope, I'd literally be able to see the top of the tower from my deck, I suspect the Tower and Ground frequencies aren't coming from the tower itself, I suspect they're are being transmitted from that other antenna cluster (although it does resemble an RCAG site).

 

[Ubbe]

Expect to pay $100 or more for a good pre-amp.

One of my first preamps where a SSB LNA-2000, or if was LNA-3000, bought for $150 more than 20 years ago and had a noise figure of 2dB and IP3 of 30dBm and are outperformed by todays PGA103+ amplifiers with less noise and higher IP3 that you can get for $20.
50M-4GHz LNA,PGA-103 + E EMI Magnetic Field Probe Signal Amplifier Preampli T5S3 | eBay
PGA103+ Ultra Low Noise Wideband LNA preamplifier RTL SDR HAM Radio | eBay

/Ubbe

 

[dlwtrunked]

One of my first preamps where a SSB LNA-2000, or if was LNA-3000, bought for $150 more than 20 years ago and had a noise figure of 2dB and IP3 of 30dBm and are outperformed by todays PGA103+ amplifiers with less noise and higher IP3 that you can get for $20.
50M-4GHz LNA,PGA-103 + E EMI Magnetic Field Probe Signal Amplifier Preampli T5S3 | eBay
PGA103+ Ultra Low Noise Wideband LNA preamplifier RTL SDR HAM Radio | eBay

/Ubbe

In tests I did, the PGA-103+ did not do as well as the RF Bay LNA-1400 that I continue to use. I do agree that the PGA-103+ is likely a better choice for many users if they properly enclose it. But considering the price of an LNA-1400 on eBay currently is about $200, it does look like a much better bargain. (I am not certain that I trust the specification numbers.)

 

[MUTNAV]

Understood. My comment was referring to wherever the transmitter antenna is located. The tower or otherwise.

A lot of times (well some of them at least) the antennas on the tower were not the main TX / RX antennas, those were usually at separate transmitter and receiver sites. Are you sure there is only one other radio tower site, maybe a second one is nearby?

Thanks
Joel

 

[kd8mkg]

A lot of times (well some of them at least) the antennas on the tower were not the main TX / RX antennas, those were usually at separate transmitter and receiver sites. Are you sure there is only one other radio tower site, maybe a second one is nearby?

I think you may have quoted the wrong post, but there are other sites at the airport that have antennas. One of them follows the exact same 4-tower architecture that all of the Indy Center sites I've located follow. The other appears to have only three towers... it looks somewhat disused and unmaintained, but could definitely be the primary tower/ground site. Below are some google street view and earth images.

It's also worth noting that Dayton does not control Approach/Departure... that has been relocated to Columbus. This is a little strange to listen to, because pilots contact Columbus approach on Dayton-specific frequencies, but I digress. I hear Columbus approach loud and clear. There are quite a few RCAG sites each with their own frequency; I can hear all of them, but some are stronger than others... all are readable.

Control Tower:


This is the site that looks like it could be an Indy Center RCAG:


And this is the 3-tower site... it isn't viewable from streetview.

 

[MUTNAV]

I think you may have quoted the wrong post, but there are other sites at the airport that have antennas. One of them follows the exact same 4-tower architecture that all of the Indy Center sites I've located follow. The other appears to have only three towers... it looks somewhat disused and unmaintained, but could definitely be the primary tower/ground site. Below are some google street view and earth images.

It's also worth noting that Dayton does not control Approach/Departure... that has been relocated to Columbus. This is a little strange to listen to, because pilots contact Columbus approach on Dayton-specific frequencies, but I digress. I hear Columbus approach loud and clear. There are quite a few RCAG sites each with their own frequency; I can hear all of them, but some are stronger than others... all are readable.

Control Tower:
View attachment 130541

This is the site that looks like it could be an Indy Center RCAG:
View attachment 130542

And this is the 3-tower site... it isn't viewable from streetview.
View attachment 130543

Probably did quote the wrong post....

I thought you were suggesting the antennas on the control tower were the main ones for the airfield. My mistake.

Thanks
Joel

 

[mlfd612]

This building and this little patch of earth are the only things keeping me from receiving my local tower and ground frequencies clearly. I can hear them, but they're way down in the noise. It only opens the squelch occasionally, so even the carrier is weak.

Most of my experience is as a ham, as such, I see preamplification as a tool to be used with caution. In the ham world, my goal is to lower the noise floor as much as possible. I want my S-meter reading close to zero when no signal is present. I use my HF radio's attenuation way more frequently than I use it's preamplification. But this is in regards to HF. In the VHF/UHF world, all of the popular local repeaters are on absurdly tall towers, I can get into them with full quieting using a 1 watt handheld in my basement. As a result, I know very little about VHF/UHF preamplification. Logically and unfortunately, height is almost an enemy for airport transmitters. They want Signals aiming slightly upward toward the horizon, with coverage only in their area of concern... they don't care about ground coverage.

Would this situation benefit from a preamp after the filter, perhaps with a variable attenuator? If so, what preamp might you suggest? (Bonus points if it uses N connectors.)

Note: This line accounts for both the height of the airport's transmit antennas and my receive antenna.

View attachment 130480

DUDE!!! Can you PLEASE tell me how you did that line-of-sight projected line from antenna to transmitter like that?
I really need that with finding any obstructions for my site and distant receiver.

I have the line projected on Google Earth, and the relative heights, but how do I get the straight line through?

Thanks!

 

[prcguy]

That building will do little to nothing for receiving a station along the line you drew and I suspect if you could move a couple of blocks to the side where the building was no longer in the way, you would have about the same reception. If the signals are noisy now a preamp will probably not help much and will more than likely cause more problems unless you have a very good band pass filter in front of the preamp. The only way to make a noticeable improvement would be if you have a lot of feedline loss and you put the preamp right at the antenna, canceling out the feedline loss and lowering the system noise figure.