What about adding another 88-108 filter after the ZX60-P103LN preamp? Is the first filter working properly?
prcguy's mast mount filter/preamp/diplexer thingee
- Thread starter prcguy
- Start date
- Status
I have two of the 88-108 notch filters and they both work the same being about 6dB down at 88 and 108 and 30 to 40dB down over most of the FM broadcast band. Ideally I would like to find a band pass filter for 118 through 512MHz with low insertion loss and 60dB down at 108MHz, or a 118MHz high pass that is 60dB down at 108MHz and below.
I am doubtful about "find" for something like that versus having something made, but my realm of experience is limited versus yours. I always call or email Dale and see what he can do at 828-743-1338 or parinc1@frontier.com. I have found him to be quick on the response when I have emailed him for things.
Ubbe
Member
How much total attenuation do you have after the preamp to the receiver?
It could be that one component are faulty and self oscillate and works as a mixer. Try the preamp directly to the discones coax and then a passive 10-12dB attenuation. And then add a 88-108MHz filter after and then also try the filter before the preamp. You have tried several receivers and they all display the EXACT same intermod/overload problem at specific frequencies so that it is absolutly sure that it is the preamp to blame and not one receiver or spectrum analyzer that can't handle the signal?
Put a 20dB attenuator after the preamp in the current configuration and check that intermod is still there and doesn't change in character so that it is proven that a better preamp or filtering are needed.
/Ubbe
I played around with attenuation in my R8600 and 10dB is needed just to tame the super high noise floor and 15dB would be about right. I placed a 10dB attenuator after the amp and before the diplexer and a two way divider at the output of the diplexer would add 3 or 4dB more loss making the noise floor ok.
The amplifier is not oscillating, its being hit with high level signals creating Intermod. The FM broadcast signals showing up in the 225-400 range also show up on an HP spectrum analyzer. Changing attentuation in the R8500 from 0 to 10, 20 and 30dB lowers the Intermod signals by the amount of attenuation I add, so the receiver and spectrum analyzer are not creating the Intermod, its the preamp.
Since FM broadcast is part of the Intermod product I think better filtering below 118MHz might have a chance, but I've been down this road before and there is usually no good solution for a preamp that is being driven into overload except finding a bigger amp. Less gain would also be on my list.
The amplifier is not oscillating, its being hit with high level signals creating Intermod. The FM broadcast signals showing up in the 225-400 range also show up on an HP spectrum analyzer. Changing attentuation in the R8500 from 0 to 10, 20 and 30dB lowers the Intermod signals by the amount of attenuation I add, so the receiver and spectrum analyzer are not creating the Intermod, its the preamp.
Since FM broadcast is part of the Intermod product I think better filtering below 118MHz might have a chance, but I've been down this road before and there is usually no good solution for a preamp that is being driven into overload except finding a bigger amp. Less gain would also be on my list.
Ubbe
Member
You have some incredible high signal levels from the antenna. If the amplifier outputs 20dBm where it starts to go out of linearity and compress 1dB, that equals 2 volts rms RF level that feeds the rest of the system and you will then have an astonishing 0dBm at the preamps input from the antenna.
I have one FM broadcast filter with smd components that if I insert that it creates intermod products that where not evident without any filter. So it can be passive components in a system that creates problems, so try and run "naked" and then add one component at the time. Maybe you have a strong transmitter at the band edges 88MHz or 108MHz and need to attenuate at least 20dB at those frequencies and not just 6dB, that seems to be very little attenuating for a proper FM broadcast filter. In US you don't seem to have anything wortwhile to monitor at 70-88MHz so at least try and attenuate as much as possible at the lower FM broadcast range. Spectrum analysers are terrible receivers, even the $50.000 ones, but you seem to have tested correctly with the 8600 owns attenuator settings, if that sits before any active components in the receiver.
/Ubbe
I have one FM broadcast filter with smd components that if I insert that it creates intermod products that where not evident without any filter. So it can be passive components in a system that creates problems, so try and run "naked" and then add one component at the time. Maybe you have a strong transmitter at the band edges 88MHz or 108MHz and need to attenuate at least 20dB at those frequencies and not just 6dB, that seems to be very little attenuating for a proper FM broadcast filter. In US you don't seem to have anything wortwhile to monitor at 70-88MHz so at least try and attenuate as much as possible at the lower FM broadcast range. Spectrum analysers are terrible receivers, even the $50.000 ones, but you seem to have tested correctly with the 8600 owns attenuator settings, if that sits before any active components in the receiver.
/Ubbe
Amplifiers will create IMD well below their 1dB compression point and where I live the preamp is being hit with thousands of strong signals which is creating the high noise floor and making FM broadcast signals show up where they don't belong. For now I will probably bypass the stupid ZX60-P103LN preamp for the 118-520Mhz range and keep the other preamp for 700-3000MHz as its not creating much of a problem and does make up for my long feedline loss.
Last edited:
I see, too much of a good thing can be bad. I am curious about your results with/without that preamp, on some steady signal ATIS stations in the low 100's for civilian and 200's for military. Not having the receiver you do, I must heavily filter before adjusting a variable preamp or not amplify at all. FM & MW filters are needed at my location for the inexpensive receivers.
I would not mind the R8600, but I would prefer a 9700 if it had a RX range of 30 - 1.3 GHz. The specs say the RX is limited to the TX range and I am hoping that is not the case. I shot off an email to a buddy who advised he picked one up a week ago to see what the reality is. The 9700 manual does not advise of a wide RX outside of the amateur bands.
I would not mind the R8600, but I would prefer a 9700 if it had a RX range of 30 - 1.3 GHz. The specs say the RX is limited to the TX range and I am hoping that is not the case. I shot off an email to a buddy who advised he picked one up a week ago to see what the reality is. The 9700 manual does not advise of a wide RX outside of the amateur bands.
Ubbe
Member
It could be PIM, Passive Inter Modulation How does Intermodulation Distortion occur - RF Page that's why I suggest starting from a clean slate. A variable attenuater just in front of the amplifer and gradually increasing attenuation would make the IM product drop rapidly but the actual receive signal drops slowly in sync with the level of attenuation, if it is the amplifier that goes into IMD. It's more difficult, almost impossible, doing those kind of tests with fixed attenuators. I don't know if you have seen the other thread about a FM broadcast enthusiast that uses an Icom8600 with a 9dB yagi and says that he gets better results with an external attenuator instead of the internal one in the receiver.
/Ubbe
/Ubbe
I did a brief test of some weak VHF and UHF ATIS stations and overall reception is just slightly better with the preamp but with an annoying and changing noise floor that makes it difficult to find a usable squelch setting. I have a 75ft run of 1/2" Superflex Heliax from the antennas to the garage, then it jumpers to another 75ft run to the house where the 8600 lives. That is the main reason I'm considering preamps at the antennas, to make up for the long feedline.
The VHF/UHF Discone has an 8ft 1/2" Superflex Heliax jumper to the filter/preamp box with silver plated connectors, and the 75ft runs of Heliax also have silver plated connectors. There are a few plated connectors inside the filter box but I doubt PIM is being generated there since there are no high power transmit signals on any of the cables or connectors. I need preamps with 10-15dB less gain and 10dB more IP1/IP3.
The Icom 9700 only receives within the amateur bands, no commercial freqs.
The VHF/UHF Discone has an 8ft 1/2" Superflex Heliax jumper to the filter/preamp box with silver plated connectors, and the 75ft runs of Heliax also have silver plated connectors. There are a few plated connectors inside the filter box but I doubt PIM is being generated there since there are no high power transmit signals on any of the cables or connectors. I need preamps with 10-15dB less gain and 10dB more IP1/IP3.
The Icom 9700 only receives within the amateur bands, no commercial freqs.
Thank you for the feedback. My buddy just confirmed about the 9700 as well. I would be pleased if it covered commercial freqs and if the 7300 had two antenna ports. I would own one of each if they offered what I want. If the 9700 offered 118 MHz RX and up I would be content.
I would suggest getting the Icom 7610 which has two antenna ports and more features and performance over the 7300, then the Icom 9700 just for the VHF/UHF ham bands, then the Icom 8600 for receiving everything else.
Last edited:
As mentioned in another thread, since the MiniCircuits ZX60-P103LN turned out to be such a disappointment, I'll be ordering a MiniCircuits ZHL-1010, which is the biggest amp I can find that still qualifies as "low noise". That should be a good fit on the 118-512MHz side for making up my coax loss and still allowing a 2-way splitter downstream and hopefully without the overload and IMD problem of the current amp. I also had to order replacement Bias Tees as the current ones are only rated for 500ma and with the ZHL-1010 preamp there will be about 1amp of current run up the coax for the two preamps.
Tonight I assembled one of my Cobham COM210B military antennas that covers 30-90Mhz and connected to the VHF lo band port of my tower top preamp/filter box. I removed the stock aluminum telescoping ground plane legs and replaced with 5ft long, 1" dia metallic conduit legs for radials. My goodness, what an amazing antenna, I have more CHP and VHF low band stuff than I know what to do with. 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. Many FM broadcast stations are in the -25 to -30dBm range with one coming in at -21dBm. I could probably run a small light bulb off the amount of RF that is hitting my receiver just in the VHF lo range.
This antenna is just sitting on its ground plane legs on the roof of my house about 25ft below the tower top and I wish there was space left at the top for this thing.
Tonight I assembled one of my Cobham COM210B military antennas that covers 30-90Mhz and connected to the VHF lo band port of my tower top preamp/filter box. I removed the stock aluminum telescoping ground plane legs and replaced with 5ft long, 1" dia metallic conduit legs for radials. My goodness, what an amazing antenna, I have more CHP and VHF low band stuff than I know what to do with. 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. Many FM broadcast stations are in the -25 to -30dBm range with one coming in at -21dBm. I could probably run a small light bulb off the amount of RF that is hitting my receiver just in the VHF lo range.
This antenna is just sitting on its ground plane legs on the roof of my house about 25ft below the tower top and I wish there was space left at the top for this thing.
It must be my location. I get a bit of 42.060 out of Springfield, MO but that's about it. There just isn't much down there to listen to and no skip. Mine is hanging in a tree with the top at 30ft.
Do you think RG6 as feedline could be part of the issue?
Do you think RG6 as feedline could be part of the issue?
I don't think RG6 would degrade things too much but try running it on your roof with 50 ohm coax like I'm doing. Maybe its that top element you made? VHF lo skip has been in and out, you should be picking up something. No 6m repeaters in your hood? The COM201B also picks up CB and 10m pretty good around here.
Hah, I still think it's my environment. Can't hear what's not there.
I wonder where all the top sections went. They are always missing in auctions.
I would look for a Whistler WS-1088 or WS-1095. They both work well.
- Status
