Cable Question

[jkabush]

Hello,

I am looking at mounting a Diamond D130NJ Discone on my roof and I'm wondering what would be the best cable to use, I have about a 30FT cable run. I've looked at LMR- 400 but I'm wondering if it's really needed. What cable would you recommend? I do scan both 155mhz range and 700mhz.

Thanks

James

 

[N9JIG]

At 700 MHz. loss is pretty severe with cheap cable so LMR400 would be a better bet than something like RG55.

The bigger issue you are going to have is the N connector, they can be difficult to install so you might want to look at pre-made high-quality cable assemblies. Adapters add loss so try to get one with the N connector at one end and the BNC at the other end. If you can get away with 25 feet of coax these assemlies are pretty common from dealers, the next common length is 50 feet and unless you can raise the antenna to use that length to an advantage then you are going to have more loss with the extra cable. You could get a custom made cable but that might be costly.

 

[jcop225]

Insertion loss for 30' of cable at 770MHz

• LMR-400: 1.18dB
• LMR-240: 1.59dB
• LMR-195: 3.18dB

(This is for the solid center conductor varieties, 'Utraflex' models will have greater loss)

What are you monitoring on 700 MHz? Is it a repeater/simulcast site that you are in the coverage area of or are you outside the intended coverage area? That can make a big difference on the amount of loss you can tolerate.

I would push back on N connectors being inherently more difficult to install. There are several types of high quality 'no solder' and 'no trim' connectors from reputable manufacturers on the market now that were designed to save time for installers. They also make stripping tools designed to simplify connectorization, but they are not required and you can make all the necessary cuts with an exacto knife.

I do agree with the sentiment of getting the exact connectors you need, adapters represent additional loss and failure points not something you want in a permanent install.

 

[wtp]

RG6 (regular cable) loss is 1.69. 30 feet 750Mhz.
not too bad for the money.

 

[jkabush]

Now in the SDS200 manual, it says the following. Always use 50-ohm, RG-58, or RG-8, BNC terminated coaxial cable to connect an outdoor antenna. Would LMR400 still be the best option? Also, I've been reading that it could put a lot of stress on the BNC connector on the scanner, is this still hold turn with a base station scanner? Can I go directly from the antenna to the back of the scanner or do I need to use a patch cable (or whatever you would call it) to connect to it?

 

[jkabush]

Also, is it best to stay away for equivalent products? If so, why?

 

[popnokick]

Assuming NO transmitting happening (ever).... +1 on the RG6 suggestion for the freqs and distance spec'd by the OP. The cable itself is far simpler to work with than LMR (size and flexibility) and the connectors are a magnitude of order simpler to install and much less cost than N.

 

[N9JIG]

A good quality RG6 would work great for an SDS200, on par with LMR400. I have had great luck with RG6QS (Quad Shield) cable. The only downside is that N connectors are not available to fit RG6 AFAIK so an adapter would be needed for at least one end if not both.

I suspect that even if the RG6 has lower loss that the same length of LMR400 that you may lose that advantage with adapters at one or both ends.

One must look at the entire system of coax, connectors and adapters (if any) for loss. You may not actually notice a difference between LMR400 with N/BNC vs. RG6 with F connectors and a couple adapters. While it could be measured with good test gear in real life use it might not be noticeable.

 

[mmckenna]

Also, is it best to stay away for equivalent products? If so, why?

Yes.

Because it's really easy for someone to say something is "equivalent" on the internet.
Equivalent can mean a number of things:
Same physical size, uses the same connectors?
Same construction?
Same jacket material?
Same performance numbers?

Or is it some cheap Chinese knock-off?

Part of the true costs of installing coaxial cable is the labor. Climbing up on the roof or your attic has risks. Paying someone else to do it costs money (or beer…)

Asking what is "best" leaves a lot of questions.
Budget is a big factor. No point in someone recommending something if you cannot afford it.
No point in recommending a big stiff cable if you have to snake it through a window, door jamb, vent hole, have to make a sharp bend, etc.

Other question to ask yourself is if your needs will ever change? It's for a scanner now, but at some point will you decide to get your amateur license? Might you need/want to transmit at some point? 75Ω coax can work for that, but you might be better off getting something else if you can afford it.

Connector installation needs to be considered, too. Are you buying this pre-terminated, or are you comfortable/equipped to do your own connector installs? RG-6 is really easy to put F connectors on.
But if you use F connectors, you need to add adapters. Adapters can be problematic, especially outdoors. They can be a weak spot. They can add loss. You can do RG-6 to BNC, but they are more expensive.

RG-6 would be fine for 30 feet and a basic install.
If cost is not a big deal, the LMR-240 might be a better choice in the long term, and get a shop to do the connector install for you.

 

[prcguy]

There will also be additional loss due to the 50/75 ohm mismatch. In a purely resistive environment the additional loss would be about .2dB but since we are dealing with cable impedance that is different from the source/load and various lengths of cables that loss can be a little less or more depending on resonant length. For sure there will be different amounts of loss at different frequencies where there could be a little less loss at higher freqs and more loss at lower freqs, its a crap shoot.

If the antenna is rated 50 ohms and the receiver is rated 50 ohms then I would use 50 ohm cable. You can save a few $$ installing TV coax and then always wonder what problems you might have caused or install a good low loss 50 ohm coax now and be done.

RG6 (regular cable) loss is 1.69. 30 feet 750Mhz.
not too bad for the money.

 

[trentbob]

There's another thread going on identical to this right now and as I said rhere I use LMR 400 on my three antennas.

Also don't forget to spend a few bucks on a product called coax- seal, it's a hand moldable plastic seal for all of your connectors to keep them corrosion free.

 

[Ubbe]

A discone like Diamond D130 seems to have almost 50 ohm impedance at 155MHz and 750MHz. Higher upp it increases up to 120 ohm at 850Mhz. Don't know what receiver you have but a Uniden BCD536HP has 100 ohm impedance at 155Mhz and 50 ohm at 750MHz and 115 ohm at 800Mhz. If you monitor above 750MHz then I would says that RG6 coax are probably as good match as anything that's 50 ohm.

Modeling Commercial Amateur Radio Antennas

Estimating Commercial Antenna Performance Through NEC Modeling. NEC Models of Arrow OSJ 146/440 J-Pole, Cushcraft R7 Vertical, Diamond D130J (Icom AH7000) Discone, High Sierra Sidekick Screwdriver, and Workman WHF40 "Hamstick" Dipole Antennas

www.qsl.net

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

/Ubbe

 

[questnz]

Coax Calculator

 

[MisterLongwire]

I agree. LMR 400 costs a bit but worth every cent

 

[prcguy]

You can find deals on Ebay, I got two 1,000ft spools of Andrew (Commscope) branded LMR400 from a surplus company for $79 each. I bought a 200ft chunk of Times LMR600 for $60 and so on. New but surplus connectors for LMR cables are also cheap as are the stripping and install tools. Deals like this come and go quickly so you have to search a couple of times a day to find them.

There is nothing sweeter than using the right tools to put on a Times EZ connector then sweep testing it past 6GHz with perfect VSWR. With the correct tools you can put an EZ connector on in less than 1 minute including heating the heatshrink and its perfect.

I agree. LMR 400 costs a bit but worth every cent

 

[popnokick]

How much are "the correct tools" for those Times EZ connectors? Hopefully there are "deals" on them too? How long do you have to search to find the deals.... or are these Home Depot / Lowes items?