prcguy's mast mount filter/preamp/diplexer thingee

Ubbe

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There is no preamp on this input and even with 40ft of LMR400 from the antenna to the tower top box, then another 150ft of 1/2" Superflex Heliax to my radio, the signal levels are great.
There's 0,3dB loss in the LMR400 at 40MHz and 0,9dB in the 1/2" so only 1,2dB loss in your coax feed at 40MHz. In comparison, if someone uses 200ft of RG6 it would be a loss of 2,5dB.

/Ubbe
 

prcguy

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Right now signal levels are fine in the VHF lo range but I will have a 2-way splitter in the line sometimes that will degrade VHF lo. VHF hi and above will be fine with the preamps at the antennas.

There's 0,3dB loss in the LMR400 at 40MHz and 0,9dB in the 1/2" so only 1,2dB loss in your coax feed at 40MHz. In comparison, if someone uses 200ft of RG6 it would be a loss of 2,5dB.

/Ubbe
 

mancow

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I think you should return that antenna and get your money back.
Got the FedEx notification today then a pic from the wife. At first I didn't know whether to be concerned or excited until I saw the name.

Oh you rotten **** ****** it's s on now.

; )
 
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prcguy

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Update.....
Been a little busy to finish the latest version of the tower top filter/preamp but I did connect the new amp in line with the new FM broadcast band trap filter to my large Discone. The amp is 75ft down the coax from the antenna and 75ft from the radio so I'm not experiencing the full benefit of having the amp at the antenna, but so far it looks real good. There is no more FM broadcast stations scattered through the VHF and UHF air bands like the other amp was causing (IMD) and my noise floor is about the same as with no amp in line.

Tuning around the VHF and UHF air bands is nice and I tuned in a couple of weak but readable UHF ATIS stations with the preamp in line and with the preamp bypassed they were too weak to copy. This new MiniCircuits ZHL-1010+ amp and the MiniCircuits ZBSF-95+ FM broadcast band trap filter are working great together and there is no need for any attenuation after the amp to reduce the noise floor.

I'm making some major changes to the tower top filter/preamp like installing coaxial transfer relays which will bypass the filters and preamps when the power is removed then switch the filters and amps in line for each Discone when power is applied. This will allow me to A/B the system with and without preamps and to take the amps out of line if I'm transmitting with high power in close proximity to the Discones which might damage the preamps. I also had to buy new Bias T injectors to handled the nearly 2A of current the new amps and relays will need. The new amp gets noticeably warm, but that will help keep condensation out of the box on cold damp nights. Lots of new holes to drill and tap plus cables to make or buy before this project is complete.



Upgraded amp arrived today for the 118 to 512MHz section of my tower top filter/amp. Ever see an LNA like this that will put out a watt of power and has a heatsink? I'll try it this weekend but can't mount it permanently in the box until the new 88-108MHz notch filter arrives next week.

View attachment 72435
 

prcguy

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Another update.....
I was a little concerned about a UHF repeater on site feeding 40W to a Yagi antenna only 6ft from the receive Discone with the new amp in line. I measured -2dBm into the preamp when the repeater is keyed up, which won't damage anything but its a whopping big signal for any preamp to deal with. I find I can listen to weak VHF air, 2m amateur, UHF air and even UHF amateur stations just 500KHz away from the repeater transmit frequency when the repeater is keyed up with no detectable degradation in reception. That's impressive and it also says a lot about the Icom R8600 I have connected to the Discone and preamp.
 

kayn1n32008

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Another update.....
I was a little concerned about a UHF repeater on site feeding 40W to a Yagi antenna only 6ft from the receive Discone with the new amp in line. I measured -2dBm into the preamp when the repeater is keyed up, which won't damage anything but its a whopping big signal for any preamp to deal with. I find I can listen to weak VHF air, 2m amateur, UHF air and even UHF amateur stations just 500KHz away from the repeater transmit frequency when the repeater is keyed up with no detectable degradation in reception. That's impressive and it also says a lot about the Icom R8600 I have connected to the Discone and preamp.
That’s impressive.


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prcguy

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Finally got my last cables for this project and also got home from some travel, so here is (this weeks) final configuration of my tower top filter/preamp box. The new high level amp I'll be using for the 118 to 520Mhz range has worked out very well so far and here is a picture of the completely reworked box.

I would have used a different component layout but the input/output connectors were already mounted from the last iteration, so I made the best use of what had already been installed. The nice and expensive Microlab diplexer is gone and replaced by a home made diplexer using a high pass/low pass filer on an SMA T adapter. It looks great from DC through about 1300MHz which was my goal and there is a little VSWR ripple in the 1.3GHz to 3Ghz range due to the 1/4" or so of connector between each filter. That might come into play when the preamps are bypassed, but when the preamps are in line it won't matter. My goal was reasonable insertion loss and VSWR up through the 23cm amateur band with preamps bypassed and it makes that.

I had just enough .141 semi rigid coax assemblies to wire the complete path in the amplifier bypass mode, giving the lowest loss. When the preamps are switched in line most coax is RG-316 Teflon and the guys at americancoax1 on Ebay came through with some great custom cables at a very cheap price. The bias Tees had to be upgraded to handle the now 1.5 amps at 12V needed to power the new amps and power hungry relays. The box also gets quite warm inside but that will help keep condensation and corrosion down as I'm not far from the ocean. I think most of the heat is from the relays.

Later today the box will be on the tower a few feet away from the antennas and I'll be scanning the bands again with nearly continuous coverage from about 25Mhz past 3GHZ, minus the FM broadcast band and a little span around 600Mhz where the home made diplexer crosses over. There is enough signal gain to make up for my worst cable loss at 3GHz and to feed a two way broad band divider so I can feed two listening areas. So far no Intermod and my noise floor is still ok.

Looking at the picture, top left is 700MHz-3GHz input, top right is 118-512Mhz input, bottom right is 25-90Mhz input and bottom left is combined output and DC power input. The 25-90Mhz range is not amplified. Not sure what this thing would have cost if I bought all the components new, but there are six filters in there not including the little military diplexer for the 25-512MHz range under the green bracket.

tower 1.JPG
 
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JSTARS03

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I think I want to do a similar setup for just the 118-512 portion. Which Bias T's did you get?
I cannot read the labels on that picture.
Thank you
 

prcguy

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I got these from China, mainly due to the current requirements. I have a bunch of different ones from MiniCircuits and other companies that are very good but for maybe 500ma tops. Coaxial Bias Tee 10MHz-6GHz 2A 50V Broadband Radio Frequency Microware Coaxial | eBay

For just the 118 to 512MHz range you can't go wrong with the MiniCircuits ZHL-1010+ amplifier. It seems bullet proof in the Intermod arena and a nice amount of gain with reasonable noise figure. You probably read my problems with other amps, some with great NF but if they can't handle the signal levels, other specs don't matter. I would also highly recommend the MiniCircuits NBSF-95-N+ FM trap filter. Its the best I've tested so far.

I think I want to do a similar setup for just the 118-512 portion. Which Bias T's did you get?
I cannot read the labels on that picture.
Thank you
 
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Ubbe

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Bias-T for 118MHz-512MHz aren't critical at all when it doesn't need to handle very low or very high frequencies.

Take a metal box that the coax go through and insert a ceramic capacitor, 47pF to 470pF, to the scanner side to isolate the DC voltage. Then connect the antenna side to a choke coil and the coil are then feed with the bias-T voltage you require. The ampere it can handle depends of the thickness of the wire used in the coil. Make sure that the power source are short circuit protected and/or have a fuse protection. The choke coil can be selfmade by winding a stiff wire around a pen. An inductance of 220nH to 1000nH will be enough and 10-20 turns around a pen can be used.
https://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Bias_t.png/250px-Bias_t.png

/Ubbe
 

prcguy

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Been using the tower top filter/preamp for several days and I'm very pleased with it so far. I have a broad band 2-way divider at the bias T at the halfway point in the coax to feed a receiver in a different area and still have plenty of gain to play with. So far no Intermod or other problems and reception is better than I've ever had for a super broad band antenna system.

I was just looking at the 900MHz amateur band with the preamp on then bypassed and its pretty dramatic. I have about 150ft of 1/2" Superflex Heliax at this receiving location and with the preamp bypassed the band looks fairly dead. Turn on the power up the coax to the tower top preamp and the band scope comes alive with some amateur repeaters and tons of digital bleeps, frequency hopping systems and other junk. My reception here at the end of my long coax runs should be like having the receiver at the top of the tower connected to the antenna with a 6ft RG-214 jumper. You can't get much better than that.

My noise floor is fine and the squelch might have to be up a notch or two at the most with the preamp on vs off. I had an idea to use a switchable preamp power up the coax from the R8600 and tap off with another bias T to feed a solid state relay that will switch the higher current for the tower top preamp. That would be great to be able to turn the preamp on or off from the radio or with the radio power button, but it seems there is no preamp power feature on the Icom R8600. Another Icom sitting next to my 8600 has that feature, but its a transceiver and would never be connected to the preamp/filter system. Oh well.

I also found a mistake in the wiring from the last picture I posted where the 25 to 90Mhz antenna was completely cut out of the system when preamp power is turned off and the preamps are bypassed. I had to switch a few cables around and now the 25-90MHz antenna passes through with the other Discones when the preamps are bypassed. I now have an antenna system worthy of a top notch receiver like the Icom R8600 that can feed anywhere in the house or garage without loosing anything in long coax runs.
 

Ubbe

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Your 150ft coax attenuates 5dB @900mhz and a 3dB gain equals to double up on the antenna. If you have a single dipole antenna and then make it a 2-stack it will be at most a 3dB gain and if you stack 4 dipoles together it will be at most a 6dB gain. So you can imagine the difference if you suddenly have no loss in the coax and then some extra dB on top of that from the 10dB gain amplifier. So even at the most problematic RF enviroment there will be a huge improvement to use an amplifer instead of going with even more expensive coax, if you only need to receive and not transmit.

/Ubbe
 

prcguy

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My 700MHz trough 3GHz antenna is a Discone. There are no gain type antennas that cover the range.

Your 150ft coax attenuates 5dB @900mhz and a 3dB gain equals to double up on the antenna. If you have a single dipole antenna and then make it a 2-stack it will be at most a 3dB gain and if you stack 4 dipoles together it will be at most a 6dB gain. So you can imagine the difference if you suddenly have no loss in the coax and then some extra dB on top of that from the 10dB gain amplifier. So even at the most problematic RF enviroment there will be a huge improvement to use an amplifer instead of going with even more expensive coax, if you only need to receive and not transmit.

/Ubbe
 

zz0468

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Upgraded amp arrived today for the 118 to 512MHz section of my tower top filter/amp. Ever see an LNA like this that will put out a watt of power and has a heatsink? I'll try it this weekend but can't mount it permanently in the box until the new 88-108MHz notch filter arrives next week.
I've seen an Angle Linear preamp oscillate and put out a 2 GHz signal at about +30 dBm. But that was a one-off failure mode. That's a really beefy preamp. I like. I built a similar project to yours a couple of years ago, except it's rack mounted indoors. I'm seriously considering a retrofit with the Mini-Circuits high level pramp.
 

Ubbe

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I'm seriously considering a retrofit with the Mini-Circuits high level pramp.
But keep in mind that it has a 3.5dB noise figure. The 1dB compression point are at +20dBm input. A $30 PGA103+ based amplifier has its 1dB compression point at 0dBm input but have instead a much better 0.5dB noise figure. If you have antenna signals that are less than 0dBm you would grab signals 3dB lower down in the noise with the PGA103+ and also save a lot of money. The high level amp are intended for extreme situations.

/Ubbe
 

zz0468

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But keep in mind that it has a 3.5dB noise figure. The 1dB compression point are at +20dBm input. A $30 PGA103+ based amplifier has its 1dB compression point at 0dBm input but have instead a much better 0.5dB noise figure. If you have antenna signals that are less than 0dBm you would grab signals 3dB lower down in the noise with the PGA103+ and also save a lot of money. The high level amp are intended for extreme situations.
Yes, absolutely correct, all of it. But...

Here in Southern California, the RF environment is brutal. I'm about 200' from a cell site built into a church steeple, 3 miles LOS from a major site with FM and TV broadcasting along with tons of Part 90 and Part 97 stuff, 1 mile from a public safety site with about 50 transmitters in it, and 1000' below the flight path of a local GA airport nearby. A preamp with a 0 dBm compression point won't cut it.

Some portions of the spectrum here, even behind a filter, have a noise floor that exceeds that of a 3.5 dB NF preamp. In that case, a 0.5 dB NF does you no good. It's external noise that that sets the site noise temperature, not the preamp.

To get it right, one must custom design for the specific site conditions. In my case, overload is a bigger consideration over low noise figure.
 

RFI-EMI-GUY

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Please keep us informed of the mods and parts list you end up with. A schematic would be nice! I have built some dedicated preamp boxes for the milsat band and for wideband scanning but see the need for more preselection and better FM broadcast supression.

Think about some remote BITE in the form of a coupler and a small test oscillator to check your gain and/or NF now and then.

Back in the early 80's (way before TX-RX somehow stole my idea), I submitted a patent application through my employer for a remote test duplex sensitivity port for Tower Mount Preamplifiers to test duplex sensitivity of 800 MHz TTA. In fact I may have built and installed the very first test port on a (750 ft) tower. At the time TX-RX simply did a gain loop back test. I added a directional coupler, and a SPDT relay with termination.
 
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