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prcguy's mast mount filter/preamp/diplexer thingee

prcguy

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I've been wanting to provide my Icom R8600 with a really good wide band VHF/UHF antenna system and I finally have a plan coming together. The only way to get the bandwidth with no gaps is with Discones and one Discone can't possibly cover the 100Mhz to 3GHz capability of the radio, so I'm using two. I found Discones rated for 100 to 800MHz loose about 10dB of gain at the horizon in the 800Mhz range so I found a nice small version that covers about 400Mhz to 3GHz and receives about 10dB better at 800/900Mhz than the big one.

I also have two listening areas, one about 75 ft down the coax from the antennas and the other has about 150ft of coax, which is Andrew 1/2" Superflex Heliax in this case. Even with Heliax there will be a lot of loss at higher frequencies so I will use some (dreaded) preamps to make up for feedline and splitter loss. Using a bunch of stuff laying around and buying a few needed parts, I assembled what I hope will be a bullet proof tower top filtered and diplexed preamp.

I was originally going to use a MiniCircuits ZX60-P103LN for both Discones, but that preamp falls way off in gain above 1GHz and even from about 100MHz to 1GHz it looses about 10dB of gain. Since its about the lowest noise figure VHF/UHF amp they offer (.4dB) with reasonable IP1/IP3 I'll use it for 100-512Mhz range and a ZRL-3500 series for 700MHz on up with a higher noise figure around 2.5dB.

The higher frequency side will start with a Maxrad brand Discone that covers about 400MHz through maybe 3GHz and feeds the high freq input connector to a high pass filter that will pass signals from 3GHz down to about 700Mhz then rolls off about 40dB at 512Mhz. That feeds the ZRL-3500 preamp then the high side 694 to 2.7GHz input of a diplexer.

The low freq side will use an Astron Wireless Discone that covers about 100Mhz to over 500Mhz to the low freq input of the box, then a 550Mhz low pass filter, then a commercial 88-108Mhz notch filter then to the ZX60-P103LN preamp. That would normally feed the 80 to 520Mhz input of the diplexer, but I made a last minute change to add a 30-90MHz non amplified input for VHF lo band. The common port of the military diplexer will feed the 80-520MHz input of the main diplexer. I also put a 25Mhz high pass filter on the 30-90Mhz input as the COM201B will be within about 20ft of my main 40/80m antenna that can run near full legal limit.

The main diplexer would be excellent for most people combining a high and low frequency Discone or a Discone and seperate 700/800/900MHz antenna using a low loss feedline. Its a Microlab BK-24N with an 80-520MHz input and a 694-2700Mhz input, a perfect split for covering commercial, public service and amateur freqs. To add VHF lo band the output of the VHF/UHF ZX60-P103LN preamp feeds a military surplus diplexer that has a 30-90Mhz input and a 116 to 520Mhz input, perfect for my application. I'll be using a military broad band Cobham COM201B ground plane to cover the 30-90MHz range into a third input on the tower mounted box.

Power for the preamps will be fed up the coax to a MiniCircuits bias tee that is now operating a little beyond its current rating and I'll have to inject at least 15 volts into the coax to get 12 volts to one of the preamps designed for 12 volts. The other preamp runs on 5 volts provided by a 3 terminal linear regulator in the box. Coax runs from the Discones will be about 6ft each and maybe 40ft of LMR400 from the COM201B to the tower mounted box. I hope to have it installed within a few days and hopefully the preamps are filtered well enough to survive the high RF levels in this area. I will be swapping out the 88-108Mhz notch filter with a high pass filter with a cutoff around 110MHz once I locate one.

BTW, I was feeling a bit lazy so the construction of the box is not my best and I purchased most of the interconnect cables inside the box. I would normally use flexible .141 semi rigid but I don't have enough connectors in stock and commercially made cables are about $25 on up each. I instead used RG-316 jumpers, a few I had made and the rest I got on Ebay from americancoax1, a local So Cal mfr. I'm really happy with the quality and workmanship and most cables were in the $7 range with free shipping, and the cables were delivered the next day. Can't beat that.

Looking at the picture below the 700-3000MHz input is on the top left, the 100-520Mhz input on the top right, the 30-90Mhz input is on the bottom right and the main output/DC input is on the bottom left. Since the main diplexer will go down to almost DC on its 80-520MHz input I could put in a future input for VLF/HF using a third diplexer.

tower .jpg
 
Last edited:

KevinC

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I'm usually that guy. The single feedline down to the shack(s) has a Polyphaser on it. The Discones are 15ft below the top of the tower on 4 to 6ft arms and probably out of the direct hit zone. I am in a very low lightning area.
Ok.

I appreciate someone that understands the benefits of amplifying as close to the antenna as possible. Trying to amplify a signal that was weak to start with and is now weaker due to coax loss is sometimes a losing battle.
 

737mech

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Ok.

I appreciate someone that understands the benefits of amplifying as close to the antenna as possible. Trying to amplify a signal that was weak to start with and is now weaker due to coax loss is sometimes a losing battle.
Interesting do you have test data or results showing any difference from amp at antenna or at radio differences? I remember reading a post from prcguy that it didn't matter at some point? Maybe there's something I missed or a new test? What amp wideband would you suggest? Should I avoid the cheap china ones?
 

prcguy

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There may be unusual circumstances where you may not notice a difference between a preamp at the antenna vs the radio end but that would be rare. For most cases the preamp belongs at the antenna and you need to filter out anything unnecessary and make sure the preamp can handle whatever high levels it might see after you filtered out what you don't need.

Out of all the preamps I've tried at my location on a Discone without any band pass filtering, only one has given satisfactory results. A dozen or more have failed due to IMD and worse reception. I'm hoping my new setup will do ok. If not I have another preamp for the VHF/UHF side with about 6dB higher IP1 of around a half watt, but the noise figure is twice as bad.



Interesting do you have test data or results showing any difference from amp at antenna or at radio differences? I remember reading a post from prcguy that it didn't matter at some point? Maybe there's something I missed or a new test? What amp wideband would you suggest? Should I avoid the cheap china ones?
 

KevinC

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Interesting do you have test data or results showing any difference from amp at antenna or at radio differences? I remember reading a post from prcguy that it didn't matter at some point? Maybe there's something I missed or a new test? What amp wideband would you suggest? Should I avoid the cheap china ones?
I try to avoid recommending anything as someone always disagrees with me.

Unless you have little to no loss/noise induced by your feedline an amp at the antenna is always better. It keeps you from amplifying all the induced noise. That's why Public Safety and cellular systems use TTA's and TMA's.
 

zz0468

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It's all about noise figure. Feedline loss in front of a preamp directly adds to the noise figure of the preamp, which represents degraded signal to noise ratio that can never be recovered.

Prcguy's design is a solid and should work quite well for him.

I did a similar multicoupler to feed 10 or so receivers with 3 antennas from 30-1000 mhz last year, but it's inside rack mounted. My antenna feedlines are closer to 20 feet, 1/2" Helix, so antenna mounted preamps didn't buy me enough to bother.
 

prcguy

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But wait, there's more! Noise figure is an important goal but also keeping the preamp(s) happy and well below the point where they produce IMD, which will raise your noise floor and create "ghost signals" or copies of real signals in places they don't belong. Its really hard to find a preamp with a low noise figure and high 1dB compression point and high IP3 that will determine if it will survive connected to your antenna. That's why filtering before the preamp is necessary and key to the whole thing playing nice.

I'm taking a gamble here with the preamps I've selected as I've never used them at my location before. The 700-3000MHz side should be ok as I've filtered out nearly everything below 700Mhz including all high power TV and FM broadcast transmitters leaving only some cell sites and Wi-Fi as unwanted in band interference. My 100-520MHz side could be a problem with the only fix being an impossible preamp to find or one that is really expensive.

It's all about noise figure. Feedline loss in front of a preamp directly adds to the noise figure of the preamp, which represents degraded signal to noise ratio that can never be recovered.

Prcguy's design is a solid and should work quite well for him.

I did a similar multicoupler to feed 10 or so receivers with 3 antennas from 30-1000 mhz last year, but it's inside rack mounted. My antenna feedlines are closer to 20 feet, 1/2" Helix, so antenna mounted preamps didn't buy me enough to bother.
 

vagrant

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Here is a picture of the tiny Discone on top of the tower mounted box. I paint all of my antennas with the same grey color.
I like that darker gray. I use a lighter gray Krylon spray and the antennas/masts still visually stand out a bit. I throw a quick Krylon clear UV spray on after.

By the way, did you spray or dip that? It looks like its on there really thick, like a couple of coats. That color looks like a battleship gray.
 

prcguy

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The little Discone has a couple of thick coats of Rustoleum satin granite. I used to get Rustoleum gloss winter grey which is lighter but shiny until it gets dirty or oxidizes over time. Both are very good for blending in with the sky and taking the glint off of new aluminum or taming white fiberglass antennas. I suppose you could do a two tone cammo using the two colors and that could break up straight sharp lines of antennas sticking up on the horizon.

Grey primer is a great color and texture for antennas but its not designed to be a top coat and will deteriorate quickly outdoors. I wish winter grey came in a satin or flat version.

I like that darker gray. I use a lighter gray Krylon spray and the antennas/masts still visually stand out a bit. I throw a quick Krylon clear UV spray on after.

By the way, did you spray or dip that? It looks like its on there really thick, like a couple of coats. That color looks like a battleship gray.
 

prcguy

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I've been busy today but had a little time just now to test the tower top box in the shop at the halfway point in coax between two listening areas. The big Discone has been on the tower for awhile and I tested a bunch of distant weak VHF and UHF ATIS frequencies with the Discone wired straight through then with the VHF/UHF portion of the box in line.

The noise floor goes up more than I want in the VHF range but overall all weak signals received in AM mode are easier to copy with the filter/preamp than without. This is a good sign and will only get better when the box is mounted up on the tower making up for half the feedline loss. I didn't find any strange signals that don't belong and no FM broadcast or other modulation riding on any signals in the amateur or public service bands so far.

Both preamps have more gain than I want where 10 to 15dB would be an ideal range and these preamps have about 20 to 26dB. Using 10dB attenuation at the radio makes the noise floor easier to live with and 20dB is not too bad but a little too much. I will probably end up placing 10dB attenuators at the output of each amp to reduce the gain and that will also provide a better match to the amps and diplexer downstream. There will also be 3 to 4dB loss at a spiltter to feed the two different listening areas and that should be just right.
 

Ubbe

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All preamps have less gain the higher the frequency, if not having some internal filter, and a coax acts the same, just the opposite to what you want. Maybe you can put together a simple highpass filter that attenuates just enough at GHz to get a perfect total gain figure, and that attenuates more at lower frequencies, where IMD usually sets in more easily, and to match the gain needed at VHF/UHF.
A simple one pole filter in a metal box, or placed inside any of the preamps or bias-T boxes that seems to have room for a few small components. There are online calculators that can be used to get the proper configuration and component values to get a fairly constant impedance over the frequency range needed and the proper attenuation at different frequencies.

/Ubbe
 

prcguy

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Go the small Discone and the filter preamp box temporarily mounted on the tower next to the existing big Discone. What a disappointment on the VHF/UHF range using the MiniCircuits ZX60-P103LN preamp. Its just not suitable as a wide band preamp for scanner use. With this preamp in circuit I have a very high elevated noise floor with FM broadcast showing up all through the 225-400MHz band and other places it doesn't belong. Even with a good 88-108MHz trap filter and a 550MHz low pass filter in front of this POS, it is getting blitzed and creating lots of Intermod.

The 700-3000Mhz side is doing much better and I'm receiving some distant 900 and 1200 amateur repeaters I could not with just a radio shack type Discone at a similar height. The preamp on the 700-3000MHz side has an IP1 and IP3 rating about 2dB higher than the VHF/UHF side but there is probably less high level out of band stuff hitting it. I'll have to try my higher level, higher noise figure VHF/UHF amp or go shopping again.
 
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