Using an OTA preamp with a discone

[robman50]

I'm using RG6 quad shield coax with my D130J discone and I was wondering if adding a preamp (like the ones used for TV antennas) would help by boosting the signals to over come the loss with the coax and/or boost the weaker signals so they would come in better.

and no I don't want to change the coax. Yes I'm aware of the 50 to 75 ohms loss.

 

[ab5r]

Please advise the length of you RG6 coax. Is this a RECEIVE ONLY antenna setup? What frequencies are you try to hear? Is your antenna in low spot or valley from where you want to listen? Are the stations that you want to listen to relatively nearby? Any obstructions? These are just some factors regarding reception.

Sometimes, adding a preamp will also increase noise and interfering signals too.

 

[mmckenna]

I'm using RG6 quad shield coax with my D130J discone and I was wondering if adding a preamp (like the ones used for TV antennas) would help by boosting the signals to over come the loss with the coax and/or boost the weaker signals so they would come in better.

Yes, it can help, but it depends on how you do it.
You want the preamp close to the antenna, so it's got the strongest signal to work with. Make sure you get the type that can be power from the existing coaxial cable. Put the preamp near the base of the antenna.
It'll help overcome coaxial cable losses, however, as AB5R says, it depends on how much coaxial cable you are running.
It'll boost weaker signals, but it won't work miracles. There has to be a strong enough signal there to work with in the first place. In other words, don't expect it to make signals appear that are not already there. Discones have no gain, and adding a preamp won't change the gain numbers on the antennas.

Keep in mind that preamps will boost everything in their range. That could mean that if you have nearby cellular sites, you can boost their power to, and thus their ability to overload your receiver.

and no I don't want to change the coax. Yes I'm aware of the 50 to 75 ohms loss.

It's minimal, don't let anyone give you a hard time about it.

 

[robman50]

I forgot to measure the length of the coax, it's under 85 feet long. Probably in the 60-70 feet range. It's an RX Only setup. Sometimes I listen to VHF for the air band, local police, fire, EMS, ham band and UHF for local business and the 70cm band. The thing about VHF is that I have some very strong signals around me and some crazy strong FM stations that overload if I add to much gain. (test done with my SDRplay + LNA).
Before when I used to use the RG8U coax (had to toss it because it got damaged), I could get everything that I could get now but much stronger. Heck, I could get the mil sats on 250 MHz and now I can't get them. The 6m band also did come in better when I had the other coax.
The thing that is also different is that I have an PL259 to F adapter on my antenna.

 

[mmckenna]

Depending on what grade of cables you are using (there are different grades of RG-6 and RG-8U), the RG-8 should have had about 1dB better performance at 250MHz.

Before adding an amplifier, I'd suggest going back to the cable you had that worked well.

It'll cost more, but it'll likely work better. Better than adding an amp and having strong FM broadcast or other services wiping out your receiver. While you can take care of those with traps and filters, it adds to the cost.

70 feet of LMR-400 would be a bit pricey, but probably not more than the amp, filters, etc.

Use a short jumper of the RG-6 to make the final connection to your radio.

 

[Ubbe]

Go over all your connectors, unscrew and spray a little contact cleaner with lubrication.
It could also be that a new transmitter have been set up that de-sense your receiver. As you notice degraded low-VHF and hi-VHF it indicates that a FM broadcast transmitter might be the problem.

Get an FM trap filter to test. Whenever you use an external antenna a variable attenuator are a neccesity to investigate what happens to your reception when you start reducing the signal level. Without one you're only guessing. And a scanners antenna input impedance could go from 10 ohm to 300 ohm depending on the frequency so it doesn't matter one bit if you use 50 or 75 ohm coax.
http://forums.radioreference.com/un...ront-end-filter-measurements.html#post2942674

I use several amplifiers connected between coax and receivers in my room and they always improve reception without having them at the antenna, which would be the best way to use them. Of course I need to have a variable attenuator between amp and receiver to get the exact signal level where the sensitivity are maxed without getting overload.

/Ubbe

 

[robman50]

Go over all your connectors, unscrew and spray a little contact cleaner with lubrication.
It could also be that a new transmitter have been set up that de-sense your receiver. As you notice degraded low-VHF and hi-VHF it indicates that a FM broadcast transmitter might be the problem.

/Ubbe

All the connectors are brand new and very shiny, also we are supposed to get a new FM transmitter on 88.5 MHz that is just under 2 KM away from my location.

 

[RFI-EMI-GUY]

I'm using RG6 quad shield coax with my D130J discone and I was wondering if adding a preamp (like the ones used for TV antennas) would help by boosting the signals to over come the loss with the coax and/or boost the weaker signals so they would come in better.

and no I don't want to change the coax. Yes I'm aware of the 50 to 75 ohms loss.

I have such a setup albeit lossy 50 ohm RG142U. It has a WJ preamp. An FM trap was required between antenna and preamplifier.

Sent from my SM-T350 using Tapatalk

 

[gmclam]

Antenna, amp, splitter, coax is a system

I use a single discone to run my multitude of scanners and receivers. I am running LMR-400 equivalent cable from the antenna to a stridesburg amp & 8 way splitter. I have an FM notch and 30MHz HP filter before the amp otherwise broadcast FM could overload the amp.

On the output side of the amp I am using the lowest loss RG6 or equivalent that I could find. Since these runs are long, often through the attic, I want minimal loss there.

Using coax that has more loss is silly. Why do that? A few pennies? Not worth it. Certainly the amp is not at the antenna, but I found my solution to be as good. The run from the antenna is fairly short and with the lowest loss coax I could find. It also provided a place to install a filter that is not exposed to weather/etc.

 

[robman50]

Hmm... get a low gain preamp with a built-in FM trap?
OR
I could install a distribution amp under the antenna.

 

[RFI-EMI-GUY]

Hmm... get a low gain preamp with a built-in FM trap?
OR
I could install a distribution amp under the antenna.

I have this for you:

https://www.ebay.com/itm/163157655037?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649

Run RG6 to a splitter downstairs and feed all the receivers you wish.

 

[mule1075]

I have this for you:

https://www.ebay.com/itm/163157655037?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649

Run RG6 to a splitter downstairs and feed all the receivers you wish.

The noise figure on that is ridiculous.

 

[robman50]

If I'd get one, it'll probably be a Channel Master CM-3410 https://www.channelmaster.com/Antenna_Cable_TV_Amplifier_p/cm-3410.htm

Gain 15dB
Noise figure typically less than 2dB

 

[RFI-EMI-GUY]

The noise figure on that is ridiculous.

You are going to get that with an FM trap as its insertion loss is ADDING about 3 dB for the adjacent frequencies. So what is specs is worst case..

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[bob550]

I have used a similar setup: roof-mounted discone to approximately 50 feet of RG6 to a four-way cable distribution amplifier with a variable output. My primary goal was to boost 800 MHz frequencies. It did that well enough, but also caused overloading on at least one scanner at VHF frequencies.

 

[RFI-EMI-GUY]

I have used a similar setup: roof-mounted discone to approximately 50 feet of RG6 to a four-way cable distribution amplifier with a variable output. My primary goal was to boost 800 MHz frequencies. It did that well enough, but also caused overloading on at least one scanner at VHF frequencies.

If you can separate your 800 MHz and VHF scanning into separate receivers you can place an attenuator before the VHF scanner. The VHF signals are less attenuated by the cabling and therefore you need to knock them down.

Commercial Cable TV amplifiers have a slope adjustment so that the gain can be adjusted flat for all frequencies regardless of the coaxial cable loss being higher at high freqs and lower at low freqs.

 

[bob550]

If you can separate your 800 MHz and VHF scanning into separate receivers you can place an attenuator before the VHF scanner. The VHF signals are less attenuated by the cabling and therefore you need to knock them down.

Commercial Cable TV amplifiers have a slope adjustment so that the gain can be adjusted flat for all frequencies regardless of the coaxial cable loss being higher at high freqs and lower at low freqs.

The particular scanner subject to overloading (PRO-163) has a per-channel attenuation option that I've set for the affected frequencies. The cable amp I use only has a simple adjustable gain control.

 

[Ubbe]

All the connectors are brand new and very shiny, also we are supposed to get a new FM transmitter on 88.5 MHz that is just under 2 KM away from my location.

They usually run those for weeks prior to going on air to test coverage and fine tune things.

/Ubbe

 

[RFI-EMI-GUY]

The particular scanner subject to overloading (PRO-163) has a per-channel attenuation option that I've set for the affected frequencies. The cable amp I use only has a simple adjustable gain control.

That is a useful feature.

Sent from my SM-T350 using Tapatalk

 

[robman50]

The particular scanner subject to overloading (PRO-163) has a per-channel attenuation option that I've set for the affected frequencies. .

I have that feature on my BCD-996xt.