RG58A/U Coax for an SDS200

[Greg43545]

Will RG58A/U Coax perform well when used to connect an SDS200 to an outside Antenna?

It would be for monitoring freqs. 771.70625 and 774.04375 for a Project 25 Phase I system. The length of coax would be short - no more that 10 - 15 feet.

 

[mmckenna]

Yes, it'll work fine as long as it's short.

Mobile antennas use RG-58 and usually come with that much coaxial cable to connect between the mount/antenna and the radio.

If you were going longer, you'd probably want something better. If the signal you want to listen to is relatively strong, you won't have any issues. If you are on the very fringes of coverage, you might want to consider something a bit better, but it's not going to make a huge difference over 10-15 feet.

 

[jcop225]

Go with LMR-195 instead, same size, essentially the same cost and much lower loss at those frequencies.

You could get lower loss with bigger cable but at that length I would say it's not necessary. One major advantage of the smaller more flexible cable is you can connect directly to the scanner without additional adapters or jumper cables which add loss.

 

[Greg43545]

Someone suggested running LMR400 with scanners. They said it is about the size of RG8, but better @ UHF.

I checked it out and LMR400 is about $86.00 for a 12 foot length at Walmart. I do have some weak signals that I monitor, BUT would it be worth it at that price ?

 

[gcopter1]

15' LMR400, with the most expensive brand of connectors (both ends with BNC's), heat shrinked; $48.75 @ the Antenna Farm...

 

[jcop225]

First of all that is very overpriced for LMR400, Generally speaking I would not pay more than $3/ft. for longer assemblies of LMR400. For reference Tessco sells the raw cable for $1.02/ft.

As stated for a short length like you have smaller cable is generally fine. Some loss numbers @900mhz (the closest frequency they all had on the sheet, 800 MHz losses will be slightly lower) for 15ft. of cable not including connector loss.

RG-58 (solid center): 2.055 dB
LMR-195: 1.665 dB
LMR-400: 0.5850 dB

 

[prcguy]

If you or a friend have tools to install connectors LMR400 is about $.85/ft and reasonably good silver plated connectors are between $2.50 and $4 each depending on where you buy. That would put a 12ft length with connectors at around $15 to $18. I think thats a bit overkill and not friendly to bend for a short run for a scanner and I would opt for LMR240, which is the size of RG-8X and lower loss than LMR195.

Someone suggested running LMR400 with scanners. They said it is about the size of RG8, but better @ UHF.

I checked it out and LMR400 is about $86.00 for a 12 foot length at Walmart. I do have some weak signals that I monitor, BUT would it be worth it at that price ?

 

[Greg43545]

Thank you, I appreciate all the help. I think since I have the RG58A/U Coax, I'll go ahead and try it and see how it works. If it doesn't perform well I'll consider the LMR240. Thanks again

 

[Ubbe]

I would suggest RG6. It's the highest performance/cost ratio of coax you can get. And the $1 F-connectors can be had as twist-on that doesn't require any crimping tools. You only have to get the proper adaptors to BNC for the scanner and whatever the antenna uses. It probably also can be bought at 15ft or 20ft lenghts in stores or amazon or Ebay with F-connectors already fitted. At 800MHz and 10ft a RG6 coax has a 0,6dB attenuation.

/Ubbe

 

[scanmanmi]

Your loss at 15' is 2.3dB so you're losing almost 1/2 of the signal. LMR400 loss would be .53 dB.
If you started with 400 and went to 58 you would notice the difference. Starting out with 58 you're not going to know what you are missing.
After I got my LMR400 I wished I had gotten the stranded softer type.

Coax Calculator

 

[Kaleier1]

I use RG-6 which has less loss at 700 Mhz than RG-58 without the high price of LMR-400

 

[prcguy]

There is a right way to do everything. Just because your diesel truck might run on nasty used McDonalds french fry oil doesn't mean you should do it. Ok, if you already have 20,000 gallons of the stuff it might justify using it for awhile, like having 1,000ft spools of RG-6 laying around so you use it for your 50 ohm scanner antenna. But if you don't already have it laying around, why not do it right the first time and just get some good 50 ohm coax?

I use RG-6 which has less loss at 700 Mhz than RG-58 without the high price of LMR-400

 

[scanmanmi]

What difference does the impedance make when receiving with a scanner?

 

[jonwienke]

What difference does the impedance make when receiving with a scanner?

Impedance mismatch causes just as much loss on receive as it does on transmit.

 

[prcguy]

The exact same difference when transmitting. Each setup will be different with additional mismatch losses at some frequencies potentially going much higher than the rated coax loss when operating with a perfect match. It depends on the antenna and coax and its length operating as a system.

If you have a 50 ohm rated antenna with certain specs and feed it with 50 ohm coax it will provide predictable performance. Do the same with 75 ohm coax and it will no longer be predictable and losses will go beyond what you think. RG-6 works, and if you have some RG-6 it won't hurt anything to try it. But if your going to buy it anyway why not get the right coax?

Or look at it this way, somebody posted on RR for some advice on what coax to use. Why not give them the best advice? Or if your going to skimp, at least tell them you could use RG-6 if you don't mind some additional loss. We don't know exactly how much but there will be some. Then the OP can make the best decision instead of thinking RG-6 is great for everything, which its not.

What difference does the impedance make when receiving with a scanner?

 

[Ubbe]

What difference does the impedance make when receiving with a scanner?

Probably nothing as all scanners impedance are all over the place, as well as most antennas except discone and logperiod types.

If you have a perfect 50 ohm impedance and connect to a perfect 75 ohm impedance, the mismatch will cost you 0,2dB.

Depending of the antenna type it could probably have something like a 20 ohm to 200 ohm impedance within it's advertised frequency range. The same goes for a scanners impedance.
Here's what the measurements are of a yagi antenna designed for 420-450MHz. 17el PMR446 446Mhz (420MHz - 452MHz) Yagi Rear Mount

And here's a Sirio 5/8 wave GP antenna @123MHz

And here are a scanner antenna advertised as 25-1000MHz

And finally a discone that doesn't swing so much in it's impedance but still goes between 30 ohm and 300 ohm, in the green plot.

BCD536HP Front-end filter measurements

I sweept the antenna input of my BCD536HP to look at the impedance and filter characteristics. It seems to be 7 filters, some are 2-pole and others 3-pole. Component tolerences are probably 5% at best so at 400MHz it could be a +/-20MHz difference between different BCD536HP scanners. Some...

forums.radioreference.com

As you see there's no point in trying to stick with a 50 ohm coax, thinking it will have an advantage over a 75 ohm coax.
There's also a reason to why all professional RX installations use 75 ohm coax instead of 50 ohm. It is the most effecient impedance for transporting low level signals going from low video frequencies for surveilliance cameras to SHF frequencies at 2000MHz for satellite receivers.

/Ubbe

 

[prcguy]

It is the most efficient when the entire receiving system is 75 ohms. However in professional/commercial satellite receive systems without LNBs and just LNAs, the feedline is always 50 ohm. 75 ohm is used more often in consumer equipment where cost is a factor.

For a 50 ohm rated antenna and a 50 ohm rated receiver, 50 ohm coax is the correct impedance to use. Why not do things once and do it right, then you will never have to wonder or worry about it.

As you see there's no point in trying to stick with a 50 ohm coax, thinking it will have an advantage over a 75 ohm coax.
There's also a reason to why all professional RX installations use 75 ohm coax instead of 50 ohm. It is the most effecient impedance for transporting low level signals going from low video frequencies for surveilliance cameras to SHF frequencies at 2000MHz for satellite receivers.

/Ubbe

 

[Ubbe]

The antenna and scanner may be rated at 50 Ohm but what you see from the measurements it is only valid at a specific frequency, at both scanner and antenna. So it is appropriate to aim for a 50 ohm coax using a 2-way radio working at one frequency and that also transmits at where the optimum impedance are 30 Ohm and the 50 Ohm value where choosen as a compromise to work at both RX and TX.

The average impedance when scanning seems to be higher and 75 Ohm are probably the best coax impedance for scanner use, also considering the cost and easy of doing your own cables without any costly special tools.

/Ubbe

 

[jonwienke]

I don't think you're factoring in the fact that a scanner's input impedance changes depending on the band it's tuned to. It's closer to 50 ohms near the frequency being received, and higher in other bands. If you sweep the input with it tuned to 27MHz, you'll get a different result than if it's tuned to 800MHz.

Given that, the average impedance being higher is irrelevant. It's higher in bands being filtered out.

 

[Ubbe]

The link to the other thread explains the scanner filters. It is different filters for different bands in a scanner. The more costly scanners have more filters that are less wide, you'll want as narrow filter as you can get. But it is typicly one wide filter for each frequency band. Even in a hi-tier scanner like Unidens x36 there's one fixed filter for 760-1300MHz and it's complicated and expensive to make a wide bandpass filter like that to have a constant 50 Ohm over its full range. Usually you sacrifise the Q value to get it work over a wider range but it also makes it less efficient and it has to be a compromise. Impedance matching between a scanner and the coax and antenna are extremly hard to do and using a 50 or 75 ohm coax are of less importance. Between 700MHz and 900MHz in a x36 scanner the impedance varies between 30 Ohm and 115 Ohm. It's probably not a 100% accurate measurement as the SWR trows the readings off but it shows that a constant impedance are hard to achive in a scanner. The best result where done with the BC780 that uses varicap filters in Hi-VHF that activly tunes the filter to the monitored frequency.

/Ubbe