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eHam Forums => VHF / UHF => Topic started by: W0DLM on July 19, 2017, 07:40:01 AM



Title: Coax For Receive-Only
Post by: W0DLM on July 19, 2017, 07:40:01 AM
Setting up a receive-only system for the 2 meter band. Coax run will be very near 100 feet. How important is it to use low-loss coax in a receive-only system? Thinking of using Davis Buryflex, but wondering if the 213 that I already have will work just as well for receive-only.

Did I mention this is for RECEIVE-ONLY!?! (Even though I'm sure I'll get some responses that talk about how much better low-loss is for transmitting.)


Title: RE: Coax For Receive-Only
Post by: W9IQ on July 19, 2017, 08:12:35 AM
The losses for receiving are the same as they are for transmitting. The question becomes how strong the signals are from your antenna that you will be receiving. If it is a close by FM repeater, then perhaps you can tolerate a few dB of loss. If you are trying to listen to the ISS or a satellite, then you probably want to avoid every dB of loss that you can.

Many times it is an economical decision. In that case, pick the cable with the lowest losses based on your budget - keeping in mind any other requirements such as burying, working around rotors, etc.

There are many sources on the internet that allow you to compare losses of various type and brands of coax cable.

- Glenn W9IQ


Title: RE: Coax For Receive-Only
Post by: N4UE on July 19, 2017, 10:43:07 AM
Like Glenn stated, loss is loss. One way out, is to use the coax you have (213 is good stuff, normally) and use a preamp at the actual antenna.
On 6M, 2M, and 70 CM, I use CATV hardline. The 6M antenna is up 100' and doesn't need a preamp. I also run high power on 6M.
On 2M and 70 CM, I use mast mounted preamps. Even with hardline, the preamps really dig down into the background.
Who made your 213?

ron
N4UE


Title: RE: Coax For Receive-Only
Post by: N8EKT on July 19, 2017, 04:20:19 PM
I have always used RG6 for all scanner and receive only applications

It cost me 7 cents a foot, has lower loss than RG213 and preamps, splitters, connectors, and adapters are a fraction of the cost of 50 ohm versions


Title: RE: Coax For Receive-Only
Post by: VA3VF on July 19, 2017, 05:50:52 PM
I have always used RG6 for all scanner and receive only applications

It cost me 7 cents a foot, has lower loss than RG213 and preamps, splitters, connectors, and adapters are a fraction of the cost of 50 ohm versions


I'll second that. It's the cable type I've been using for the past 3 or 4 years. I use it also on HF, both RX and TX.

73 de Vince, VA3VF


Title: RE: Coax For Receive-Only
Post by: W1VT on July 20, 2017, 06:44:38 AM
The big factor is the background noise.  If you are really lucky, the background noise is around 300 Kelvin.  If not, the background noise can be much higher than that.  The standard test is to listen on AM/SSB/CW and see if the noise level increases when you hook up the antenna.  If you want to be more rigorous you would compare the noise level against that of a dummy load at room temperature.

If you have a really quiet location and want to use a really long run of coax, it may be more cost effective to put a receive preamplifier at the antenna and use cheap coax, like RG-6.  But many cheap preamps won't work.  The problem isn't the sensitivity of the preamp.  It is the close proximity of local transmitters (cell phone sites, pagers) and the lack of filtering to remove those signals.  Preamps can't work if overloaded by nearby strong signals.  SSB Electronics does sell preamplifiers with filtering.

Zack W1VT


Title: RE: Coax For Receive-Only
Post by: W0DLM on July 20, 2017, 07:05:55 AM
It was kind of my guess that loss was loss, regardless of tx or rx. Thanks for confirming that.


Title: RE: Coax For Receive-Only
Post by: W1VT on July 20, 2017, 11:55:01 AM
Sensitivity calculations factoring in antenna noise pickup are typically done for EME stations.  You could use that same methodology at other frequencies just by plugging in more realistic numbers at the frequency of interest.  Coax loss typically doesn't matter at HF because the background noise dominates.  On bands like 432 MHz, coax loss matters a great deal, because with optimally designed antennas, the background noise when the antenna is pointed cold sky can be very low. 

Zack W1VT


Title: RE: Coax For Receive-Only
Post by: N4UE on July 20, 2017, 01:44:58 PM
It was kind of my guess that loss was loss, regardless of tx or rx. Thanks for confirming that.

Hi. Zack has stated this quite eloquently. Perhaps my reply should have been.......

A receive signal lost in the coax, is gone forever. Adding a preamp at the receiver is not the best choice.
The coax loss adds directly to the system noise figure.
That's why I suggested using the coax you already had (if it's decent) and adding a small preamp at the antenna. This is easily done using a DC insertion "T". I have a bunch of the inexpensive Chinese preamps. They work quite well, have a wide bandwidth, and most have a low noise figure.
However, like Zack said, you're OK as long as you don't have a close-by BCB or FM station.

I should have mentioned the RG-6 also.
I have a Radio Shack discone mounted to the side of one of my towers at the 70' level. Right at the antenna, I use a small tv type, wide band preamp. I think the preamp is also from RS, but I believe the same model is sold at Lowes. It actually fits inside the pipe that is used to mount the antenna.
The DC power comes from a wall wart in the shack via the "T".
The antenna feeds RG-6. The connectors are very easy to attach and the tools to make it all work are cheap and everywhere. Although the Quad 4 stuff may be better, I find it's a pain to work with, and even takes it's own connectors.
One last thing.....the 'flooded' cable is more water resistant, but is sure is messy!

ron
N4UE


Title: RE: Coax For Receive-Only
Post by: WA8ZTZ on July 21, 2017, 05:04:34 AM
Another vote for RG-6.


Title: RE: Coax For Receive-Only
Post by: W0DLM on July 21, 2017, 11:01:57 AM
Okay, if I had to buy new cable, I get that rg-6 is way less expensive than either 213 or Buryflex. But I already have plenty of 213 laying around. The difference in attenuation characteristics between rg-6 and 213 look to be extremely minimal. So it would make more sense for me to use the 213 that I have, over buying new rg-6. Or am I missing something?

Also, the mention of cheap preamps being available. Is this the sort of thing you're talking about?
http://tinyurl.com/y7f3vbuu


Title: RE: Coax For Receive-Only
Post by: VA3VF on July 21, 2017, 11:05:15 AM
Okay, if I had to buy new cable, I get that rg-6 is way less expensive than either 213 or Buryflex. But I already have plenty of 213 laying around. The difference in attenuation characteristics between rg-6 and 213 look to be extremely minimal. So it would make more sense for me to use the 213 that I have, over buying new rg-6. Or am I missing something?

Also, the mention of cheap preamps being available. Is this the sort of thing you're talking about?
http://tinyurl.com/y7f3vbuu

If terminating 213 is as easy and cheap as RG-6, then yes, use it. I don't use RG-6 quad shield, they are a pain.

Yes, this is the board I purchased, but have not used it yet, so cannot comment on performance.


Title: RE: Coax For Receive-Only
Post by: N4UE on July 21, 2017, 12:20:48 PM
DLM, I also use similar preamps. They are all over eBay. I use them for counter preamps, etc.
The connectors (SMA) are quite nice and I've purchased a kit to make my own coax jumpers using RG-316/RG-174.
Plus, you can buy SMA to (any possible connector) very cheaply.
Have fun and let us know how you work it out....

ron
N4UE


Title: RE: Coax For Receive-Only
Post by: KQ6EA on July 24, 2017, 02:02:03 PM
Another vote for RG6. Very low loss, and CHEEP! I bought a 1000' spool of Belden RG6QS when I rewired the house, and had a lot of it left over. I've been using it for all my receive only stuff (wxsat, scanner discone) and it works great

I've never had any problems at all terminating RG6QS, which is all I run. If you have the correct stripper (about $20 at Solid Signal) and the correct connectors, you can strip and install the connector in less than 5 minutes.

]73, JIm


Title: RE: Coax For Receive-Only
Post by: KA9UCN on July 24, 2017, 07:01:09 PM
RG6
Joe KA9UCN


Title: RE: Coax For Receive-Only
Post by: K1ZJH on August 03, 2017, 04:33:08 AM
Another vote for RG-6,  but good quality stuff!  Also a vote for Thomas and Betts F connectors.  They require a special tool for installation, but they are far better than the CATV stuff sold in retail stores.

Pete


Title: RE: Coax For Receive-Only
Post by: VA3VF on August 03, 2017, 04:39:52 AM
Also a vote for Thomas and Betts F connectors.  They require a special tool for installation, but they are far better than the CATV stuff sold in retail stores.

Pete

+1...money well spent.


Title: RE: Coax For Receive-Only
Post by: W2WDX on September 19, 2017, 11:14:15 PM
Now see ... I went the opposite route. I live in the Bronx of NYC. Radio is all over the place. If you are in the city, your are near some transmitter. Always.

I live on the ninth floor of a 24 story co-op. I had to run coaxial through conduit to the roof. I was only allowed by the building to run one coax cable and two control cables. So I ended up going with Times Microwave LMR-1700. Crazy right! Not so.

The money was well spent. On most bands my losses are down near well matched open-wire line. Well below 1 db, a little over 1.6db on 70cm. On HF it's almost as if I have 1' of coax instead of over 250'. UHF is the only band where I wanted to use an antenna pre-amp, and it's only 6db in gain. (I also used one of those cheap china made do-dads). I also run QRO up to UHF many times (only when it's needed) so I needed the power handling.

However, I recognize this kind of expense was only justified from my specific application and not something most Hams would do. However, the losses on receive at VHF/UHF should never be taken lightly ... they can be significant and make the difference between hearing just noise or picking out the really weak signals.


Title: RE: Coax For Receive-Only
Post by: AA2UK on September 20, 2017, 09:54:30 AM
Sure RG-6 seems good if you're listening to VHF FM otherwise I find it utterly useless for moderate to serious VHF weak signal work. There's also the impedance mismatch.
As far as the EBay the Chineese LNA's are junk if not properly filtered as I believe most if not all are not filtered.
While on the subject cross-polarization of antennas results in >20db performance hit.
Bill AA2UK


Title: RE: Coax For Receive-Only
Post by: KD8IIC on September 21, 2017, 07:14:51 AM
 RG-6 or ladder line.


Title: RE: Coax For Receive-Only
Post by: W2WDX on October 06, 2017, 01:14:37 PM
Here here, Bill. Filtering that LNA was a no-brainer once I saw it. It's noise spec and gain was spot on though. I just wanted the cheap amplifier board pre-assembled. A filter I could build, SMT components not so much for this OM.