Signal Amplifiers

[N0ZPK]

I found a couple of signal amplifiers in a junk box I had, I want to use a discone antenna for receiving air and milcomms. I am wanting to know if these will work cause I know a discone has no gain.I haven’t been able to find much data about them, here’s a couple of pics I took, the Avantek specs are UTA-568M 5-1000MHZ @24VDC, and if they are good to use should they be mounted close to the antenna or close to the receiver, coax is LMR400.. Hopefully they will show.
Thanks for any help,

 

[737mech]

Amps

You can try but I doubt you need amps in PHX? I'm thinking you can't hear stuff you think you should hear maybe it's because you actually need a trap? Most common traps to open scanners to airband and milair is the FM notch trap. Radioshack made a cheap one for TV users but works ok. There's another one from rtl/sdr here's the link https://www.amazon.com/Broadcast-FM...ocphy=9030815&hvtargid=pla-570136450999&psc=1 Not too expensive but works. If you are really close to the mountain with all the antennas on it near the south side of town you may need a better trap from parelectronics.com ? Hope this helps? Just to add the coax used should be lmr-400 or belden 9913 something like that for long runs over 100 feet between scanner and antenna. Less than 50 feet no worries use the regular rg-6.

 

[Thunderknight]

Amps should almost always be mounted as close to the antenna as possible. The idea is to amplify the signal as soon as it's received and then send it down the coax. If you put it at the receiver, you are just amplifying noise and loss from the cable.
But what 737mech said is correct anywhere you have a crowded RF environment. You can always stack a filter/trap and an amp...if you do that, put the filter BEFORE the amp (towards the antenna)...so you filter the strong unwanted signal before amplifying the desired signal going down the coax.

 

[zz0468]

You can try them, but I don't think the noise figure on either of them is any better than the receiver it might be feeding. I have one of those Avantek amps, and the NF measures about 6 dB.

In order for a preamp to actually improve weak signal reception, the noise figure MUST be lower than the receiver that follows it. If not, it will make strong signals stronger, and weak signals noisier. It will also reduce the overload tolerance of the receiver, if there's too much gain. Sometimes an attenuator is needed between the preamp and the receiver.

Some situations might make it usable... In front of a splitter to drive multiple receivers, it could essentially maintain the receiver noise figure, or at least keep the splitter loss from making it much worse.

Bottom line... Try it. But if you want to drag real weak ones out of the mud, there are better choices.

 

[ko6jw_2]

Several things to keep in mind about aircraft monitoring. The first is that it will almost always be line of sight. The aircraft is at a high altitude and operates at relatively low power. Second, discones are unity gain, but they also offer broadband coverage and flat impedance. This is important because aircraft bands are quite wide. Next, antenna gain is achieved by lowering the radiation angle. Great for two way land mobile, not so good for aircraft because they are up in the air. Last amplifiers amplify everything: noise, out of band signals, internod and they can overload scanners which don't have commercial quality receivers. Overloads will desensitize receivers.

The point is that we all believe that there must be weals signals the we could hear if only we had more sensitivity, but that isn't always true.

Antenna height and low loss cable are the keys. Height is not even that important for aircraft reception if the antenna is clear of obstructions.

I once worked an aeronautical mobile station on simplex at 250+ miles with 5 watts from my car.

If you're going to use an amp, by all means put it at the antenna. Don't try for maximum gain, just use it to overcome loss in the feed line.

 

[Ubbe]

Don't bother spending hours on those amplifiers. Get a $30 one with 0.7dB noise figure or any other low noise model from the same company.
https://www.ebay.com/itm/Ultra-Low-...Tee-3-3-10V-RTL-SDR-LNA-NF-0-7dB/282939793263

Scanners have something like a 6dB noise figure and using an amplifier with less noise than that will improve your sensitivity the same amount of the dB as the difference are in their noise figures.
Using LMR400 means it probably doesn't have that much attenuation that it makes a big difference, at mil-air frequencies, where you put the amplifier but if it's easy to do then do install it at the antenna end.

If you have a 20dB amplifier you probably want to attenuate the signal going into the scanner by 10-15dB to not get de-sense or interference. A variable 0-20dB attenuator are prefered to set the exact right level at all frequencies. A discone works best for FM broadcast frequencies so a FM trap filter migh be neccesary. A good amplifier can handle those signal levels and trap filters always attenuate all frequencies a little so install it after the amplifier and try getting a filter that doesn't attenuate into the aircraft band.

If you first try out everything at the scanner end you can see where the filter are most effective at your location with your antenna and scanner, before or after the amplifier. If your coax are of a relative short lenght then it's probably no point in moving the amplifier to the antenna. With other antenna types you have big impedance variations when you monitor over a wide frequency range and an amplifier at the antenna will work as an impedance match and improve reception. But discones already have a steady impedance over it's whole frequency range so less reason to have an amplifier mast mounted.

/Ubbe

 

[N0ZPK]

Ok, my location is in the southeast valley out in city of chandler. I do think my problem is I am in a somewhat “hole” here. I do want to thank everyone for some ideas and will see what happens when I change a few things around and will let everyone know how it goes.....
Thanks,

 

[nanZor]

A good amplifier can handle those signal levels and trap filters always attenuate all frequencies a little so install it after the amplifier and try getting a filter that doesn't attenuate into the aircraft band.

Insertion loss and a bit of attenuation inside the band you are having problems with (within reason), can be ok if it restores proper operation of the receiver itself that is actually being desensed.

In other words, if the filter restores proper operation, but leaves a little bit of it's own attenuation behind, that is far better than a receiver that is totally unusable without it.

The temptation is to bench race a spec - one always has to test *if* they actually have a desense issue.