will a 30 foot reduction in coax length make a noticeable difference?

[mindheavy]

I am new to SDRs, still learning quite a bit about radio.

I have been monitoring local public safety channels on the 800Mhz frequency range, a P25 system.

I'm currently using an rtl-sdr V3 and sdrtrunk. The setup is working well but I have a couple of changes I would like to make.

I live outside of town, 9 miles line of sight from the transmitting tower. I have an antenna mast that was on this home when we purchased and can get the antenna above surrounding trees for a clear shot into town.

I bought a yagi antenna from ebay which is a browning BR-6386 with a type-n fitting and 9-db gain.

I originally had the antenna at the top of my mast, 40 feet above ground so I purchased 50 feet of DX engineering 400max coax. The cable seems great and I don't really have any issues with it.

But that's where my first question comes in:

In sdrtrunk, using the above setup, I consistently get a channel power of about -18db. It seems to stay locked on to the control channel and rarely seems to drop traffic. However, I ended up moving the yagi lower on the antenna, and could probably get away with a coax length of much less, maybe 20 feet or thereabouts. How much of a difference would/could a 30' reduction in coax length be noticed?

Lastly, is there a better type of coax for my purpose (decoding P25 channel using an rtl-sdr V3) than the DX engineering 400max?

Thanks for any help.

 

[WA0CBW]

Look up the loss (at the frequencies you want to hear) and calculate the loss for 30 feet. That will tell you the amount of signal loss you will have.
Bill

 

[buddrousa]

50 foot of Belding 9913 or the LMR400 should not keep you from monitoring the site 9 miles from you. You should be able to monitor the site with an inside antenna.

 

[JeffChells]

A 30 foot reduction in coaxial cable length can significantly reduce the signal loss in your receiver system, even with a good cable like your 400MAX. It has a nice specification of 08. dB loss at 30 MHz, but it will be considerably higher at 800 MHz. You're not using an LNA mounted at the antenna, right? So, your system noise figure will be affected by that cable run, since your first active stage is after the feedline losses. A few dB of RF may not make much of a difference in your decoded local trunk signals, although you should notice the change if you listen to more distant stations or perhaps local people on FM handhelds.

 

[mmckenna]

If you are monitoring that one system, and it's decoding fine, then I'd not mess with it.

Reducing loss is usually a good thing, but if you are decoding traffic without errors, reducing feedline loss isn't going to make any difference.

Plus, you need to either buy a new coax assembly, or purchasing the tools to reterminate it yourself. If you don't have the crimpers to install connectors yourself, that'll cost you some money, plus that of the new connectors.

 

[mindheavy]

Thanks for the replies. I'll just stick with the 50' of cable for now and continue monitoring. Mostly I was just curious for ways to improve signal strength. When I drive around in town with my laptop, rtl-sdr, and the kit coax and dipole antenna it comes with, I get signal strengths around 0db up to even +1.5db.

Is it mostly the distance (9 miles) I am from the transmitting antenna that results in my -18db signal?

 

[G6FGO]

Mostly I was just curious for ways to improve signal strength

Putting the aerial back at the top is a cost free winner.

 

[sonm10]

-18 signal is really high. You might want to consider attenuation. At least lower the gain of the SDR.

 

[mmckenna]

I get signal strengths around 0db up to even +1.5db.

Is it mostly the distance (9 miles) I am from the transmitting antenna that results in my -18db signal?

If those numbers are accurate, that's -really- high signal strength and not in need of improvement. Usually anything better than -95dB is considered good in the LMR field.

 

[mindheavy]

If those numbers are accurate, that's -really- high signal strength and not in need of improvement. Usually anything better than -95dB is considered good in the LMR field.

Interesting. I'm simply going off of the channel tab in sdrtrunk. It shows a channel "Power (dB)", currently my rtl-sdr 9miles out of town sits at -20.0 with a peak of -18.0. But yeah, driving around in my downtown area near the tower it's right up next to 0dB and I've seen it go +1.5dB before.

Again, I'm quite new to all of this and only getting those numbers from that channel tab.

 

[mmckenna]

OK, so yes, I don't think you'd see any improvement with this specific system from that location.

If you were trying to listen to other distant stations/systems, then less coax may be beneficial.

 

[ArloG]

You didn't mention the SDR you're using. If it has a bias-tee setting with the ability to output 5vdc at the antenna port. A bias-tee preamp might be the hot ticket. I didn't look up your coax specs and hopefully it's comparable with 9913/LMR-400.

I'm using a Nooelec LANA with SMA>N-Type coax adapters and am pretty happy with it.
I initially used it right at the radio indoors during the winter then put it at the antenna where it should be. Moderate improvement especially at higher frequencies. It has no filtering and there is only 1 FM station here. So I cant really say if other types of LNA's are better or worse.
It is not one bit weatherproof and so is stuck in a Carlon plastic electrical outlet box with 1/2" gland fitting from LANA>radio.
At the antenna side I drilled a hole with a Unibit that let the antenna N female poke through.
A couple of hardware store rubber washers stacked on it so that when tightening the male fitting a nice snug seal is had.
And of course. My favorite clear Flex Seal spray to button it up. A couple of years up now and not a burp.

Oh. And of course, a spark gap arrestor is tucked inside with ground wire poking out to a bonding point.

 

[mmckenna]

You didn't mention the SDR you're using. If it has a bias-tee setting with the ability to output 5vdc at the antenna port. A bias-tee preamp might be the hot ticket. I didn't look up your coax specs and hopefully it's comparable with 9913/LMR-400.

Based off the signal strength readings he's getting, as well as the distance to the tower/coax/antenna gain, the last thing he needs is a preamp.