Loss with my antenna and LMR400

[Ronnierozier2]

Hello all,

Rather new to all of this after a 20 year gap in my scanner use. I just recently purchased the SDS100 to only listen to P25 traffic here in my county thats all im interested in. Most everything here is P25. My question is about Loss in relation to my setup. Can someone tell me what they think my lost is? I'll describe my setup below.

On the highest point on the back of my house which is about 35' I have a 57" omni 700Mhz 6dbi gain antenna from DPD Productions. then I have 100' of LMR400 running down the back of the house - Radius Bend - then down the side of my house - Radius bend - then down the front of my house with another - radius bend. 100' anmost got me to where i need to be but i had to add a 15' LMR400 jumper. so its 115' of LMR, couple bends and two inline lightning arrestors with a 10awg ground wire running to an 8' ground rod, one by the antenna and the other where the 100' LMR meets the 15' LMR jumper. I know thats a lot of feedline but believe me this is the absolute shortest run of feedline for where i wanted the antenna and where i wanted to put the radio. It's a large house thats why its so much feedline so shortning it is out of the question.

Looking for what you guys think my loss is and any tips to reduce loss with my current setup. again, it's no way to reduce the amount of feedline i have.

Just as an FYI: I do have a 250' roll of COMMSCOPE LDF4-50A LOL

 

[MDScanFan]

Regarding the loss of your LMR-400 run, just do the math at the frequency of interest per the datasheet for your total length. Maybe around 4 dB of loss from my estimate.

https://www.timesmicrowave.com/DataSheets/CableProducts/LMR-400.pdf

 

[prcguy]

If you look at an LMR400 loss chart its about 3.9dB for 100ft at 900MHz. That would be a little bit more loss than at 700MHz so with your extra 15ft I would say your in the 4dB loss range. You might consider a low noise figure low to modest gain preamp right at the antenna which will more than make up for the feedline loss. You would want to make sure there are no super strong signals in the 600-900MHz range that might overload the preamp.

You can power the preamp up the coax using a pair of Bias Tees and I would look for a preamp with 10-15dB gain, less than 1dB noise figure and a 1dB compression point higher than 20dBm. With something like that you could split and feed at least 2 receivers with better signal than you have now.

 

[mmckenna]

At 780MHz (towards the center of the band) you have 4.1dB of loss just in the cable. That's about 60% signal loss just in the cable.
Then add in all the connectors. If they are installed correctly, using something good like N connectors, figure 0.1dB per connector. I'm counting 7 or 8, so now you are up near 5dB of loss. Hopefully you are not connecting the LMR-400 directly to the scanner, so add in the loss of the connectors on the jumper, plus the negligible loss of the jumper cable.
Lightning protectors will add a bit of loss, also. Polyphaser shows theirs at 0.1dB of insertion loss.
I think you are getting up near 5.5dB of loss pretty easily.

But none of that really means much as long as you can hear the traffic you want to hear. If there is a strong enough signal at the antenna, there will be enough to be decoded by the radio.

If you are having problems decoding, you'd need to look at reducing your losses:
- Having two lightning arrestors isn't helping you. By code, you only need one where the coax enters the home.
- Reduce the number of connectors.
- Use the 1/2" Heliax.
- Use a higher gain antenna
- Use an amplifier at the antenna to boost what gets sent down the coax.

But, if it works, it works, and loss doesn't matter.

 

[Ronnierozier2]

If you look at an LMR400 loss chart its about 3.9dB for 100ft at 900MHz. That would be a little bit more loss than at 700MHz so with your extra 15ft I would say your in the 4dB loss range. You might consider a low noise figure low to modest gain preamp right at the antenna which will more than make up for the feedline loss. You would want to make sure there are no super strong signals in the 600-900MHz range that might overload the preamp.

You can power the preamp up the coax using a pair of Bias Tees and I would look for a preamp with 10-15dB gain, less than 1dB noise figure and a 1dB compression point higher than 20dBm. With something like that you could split and feed at least 2 receivers with better signal than you have now.

You have peaked my interest talking about a preamp. Can you share more about that? I’m interested in only 769mhz to 775mhz if that helps any. Post a link of what your talking about.

 

[Ronnierozier2]

At 780MHz (towards the center of the band) you have 4.1dB of loss just in the cable. That's about 60% signal loss just in the cable.
Then add in all the connectors. If they are installed correctly, using something good like N connectors, figure 0.1dB per connector. I'm counting 7 or 8, so now you are up near 5dB of loss. Hopefully you are not connecting the LMR-400 directly to the scanner, so add in the loss of the connectors on the jumper, plus the negligible loss of the jumper cable.
Lightning protectors will add a bit of loss, also. Polyphaser shows theirs at 0.1dB of insertion loss.
I think you are getting up near 5.5dB of loss pretty easily.

But none of that really means much as long as you can hear the traffic you want to hear. If there is a strong enough signal at the antenna, there will be enough to be decoded by the radio.

If you are having problems decoding, you'd need to look at reducing your losses:
- Having two lightning arrestors isn't helping you. By code, you only need one where the coax enters the home.
- Reduce the number of connectors.
- Use the 1/2" Heliax.
- Use a higher gain antenna
- Use an amplifier at the antenna to boost what gets sent down the coax.

But, if it works, it works, and loss doesn't matter.

its working great right now decoding traffic on three sites around me. There is a 4th site I would really like to listen to but it’s just a little out of my range. I live in the center of a small town and when I drive out to the edge of town about a mile away I can get that site on a mobile setup.

 

[prcguy]

Do you have or have access to a cheap dongle SDR to look at the spectrum around 600-900MHz to see what's out there? If you have super strong cell tower signals nearby that could make a preamp unusable.

Here would be a starting point using new components, a filter at the antenna then a preamp close by.

Filter Lumped LC High Pass Filter, 700 - 3000 MHz | NHP-700+ | Mini-Circuits

Preamp Low Noise Amplifier, 50 - 3000 MHz, 50Ω | ZX60-P103LN+ | Mini-Circuits

Bias Tee, need 2 RF DC Block Low Insertion Loss Coaxial Feed Bias Tee Module 10MHz6GHz DC1-50V | eBay

6dB attenuator, place after the preamp 2W SMA Coaxial Fixed Attenuators Frequency 6GHz 8GHz SMA Fixed Connectors | eBay

Ideally you would use a front end filter that only passed 769 to 775MHz with sharp skirts but that would be an expensive custom filter.

You have peaked my interest talking about a preamp. Can you share more about that? I’m interested in only 769mhz to 775mhz if that helps any. Post a link of what your talking about.