Antenna gain and cable loss

[JohnDistai]

I just installed a Diamond D130j in my attic, as high as I can get it. The antenna has listed a (nominal) gain of 2db (as listed on ScannerMaster). The greatest height I could indoors required a 50ft cable run. I bought a 75' cable just to be safe.

If I run a loss calculation of LMR400 between 750Mhz and 860Mhz, the numbers range between 1.7 and 2.8dB of loss.
I do receive the local airport now. I haven't noticed anything different on the 750-860 range. I'm new to antennas, so I'm not sure if I'd be able to tell a difference yet.

Did I just go through all that effort for little gain?

 

[Kiniutech]

Check here Coax Calculator

 

[AB5ID]

What antenna did you use before? Could you hear the airport before?

 

[KevinC]

I just installed a Diamond D130j in my attic, as high as I can get it. The antenna has listed a (nominal) gain of 2db (as listed on ScannerMaster). The greatest height I could indoors required a 50ft cable run. I bought a 75' cable just to be safe.

If I run a loss calculation of LMR400 between 750Mhz and 860Mhz, the numbers range between 1.7 and 2.8dB of loss.
I do receive the local airport now. I haven't noticed anything different on the 750-860 range. I'm new to antennas, so I'm not sure if I'd be able to tell a difference yet.

Did I just go through all that effort for little gain?

That "gain" is 2 dBi, which is basically zero gain.

Antenna Gain dBi vs. dBd - Ham Radio Academy

How does antenna gain stated in dBi compare to gain stated in dBd for the same antenna?

hamradioacademy.com

Also wide-band discones typically aren't the best for the 7/800 band.

 

[MUTNAV]

I don't know if that was supposed to be a pun ( "Did I go through the effort for little gain"). If you mean benefit instead of gain, and you can now receive what you want, then it was worth the effort/gain.

Even a loss of gain sometimes is worth it if you can get the antenna in a better position.

As far as the gain of a discone, it depends on the reference, it sounds like your (the companies) reference is against an imaginary "isotropic" radiator. Discones generally don't have much gain, just lots of bandwidth.

As far as the importance of gain, keep in mind that a radio with a few milliwatts can reach satellites since there is (usually) an unobstructed path.

If the antenna is receive only, and your concerned about coax loss, a low noise preamp might help IF there is a problem (there may not be).


Thanks
Joel

 

[JohnDistai]

What antenna did you use before? Could you hear the airport before?

I used the stock antenna on the radio. I could hear the airport sporadically, but it was very faint. I can hear much more of the airport now. So there was a gain there, but no real gain in the 750-850Mhz range.

This was mainly a test prior to placing the antenna outside. However, if I understand the grounding best practices correctly, the antenna coax should from the mast, down my house, into a single point ground, and then back up to the attic where my radios are. I'm estimating that this is 75 ft round trip, so it doesn't sound like there's much to gain from it (unless I get a higher gain antenna).

 

[JohnDistai]

Yes, not the best for 700-800. It's my first antenna, and I just got a Stridsberg to feed multiple scanners. Perhaps I'll try the Omni-X next, since it's a bit tighter. Ideally, I have one for Airbands, one for the 400-500, and one for 700-800. Room to grow, I guess.

 

[mmckenna]

I used the stock antenna on the radio. I could hear the airport sporadically, but it was very faint. I can hear much more of the airport now. So there was a gain there, but no real gain in the 750-850Mhz range.

Well, as stated above, the discone antenna has zero dBd of gain, which is equivalent to a dipole. You are loosing some of the received signal in the coax, and that's normal. LMR-400 is pretty good for this sort of application.

If you want better 700/800MHz reception, you might want to consider a dedicated 700/800MHz antenna, which will have some gain.

Or, add a preamplifier, and/or better coax.

The antenna on top of your scanner may not have been a good performer, but there was zero feed line loss. But down low in your home, it's going to suffer.

This was mainly a test prior to placing the antenna outside. However, if I understand the grounding best practices correctly, the antenna coax should from the mast, down my house, into a single point ground, and then back up to the attic where my radios are. I'm estimating that this is 75 ft round trip, so it doesn't sound like there's much to gain from it (unless I get a higher gain antenna).

Mounting in the attic isn't always the best choice. Some roofing materials/insulation/etc. have metal in them and that will block your signal. Even if it's just wood and shingles, there's still some attenuation. Getting the antenna outside and up as high as you safely can will likely show improvement.

As for the grounding, I think you misunderstood something along the way. There is no need to run the coax all the way down to ground level and back up to the radios in the attic. Just run the coax from the antenna to the radios. Where the coax enters the house, make sure you install your lightning suppressor. Then just run a suitable size ground conductor down to the ground rod. Make sure that ground rod is bonded to the original house ground rod, probably near your electric panel. No need to waste all that coax going down and then back up….

 

[AB5ID]

So definitely some improvement on the air band. Is there a weak signal or a system you currently can't hear on the higher bands? I guess it's a question of what are you currently receiving well and what do you need to receive better? This discone is a great wide band antenna but the compromise is low gain across that band.

 

[JohnDistai]

Mounting in the attic isn't always the best choice. Some roofing materials/insulation/etc. have metal in them and that will block your signal. Even if it's just wood and shingles, there's still some attenuation. Getting the antenna outside and up as high as you safely can will likely show improvement.

As for the grounding, I think you misunderstood something along the way. There is no need to run the coax all the way down to ground level and back up to the radios in the attic. Just run the coax from the antenna to the radios. Where the coax enters the house, make sure you install your lightning suppressor. Then just run a suitable size ground conductor down to the ground rod. Make sure that ground rod is bonded to the original house ground rod, probably near your electric panel. No need to waste all that coax going down and then back up….

Perhaps I am misunderstanding something on the ground. Any of the grounding-related information seems aimed at Ham sites, so I figured that included scanners. They're talking about perimeter grounds, single point ground panels, common-grounded power at the same spot, etc.

I'm not sure I understand what a lightning suppressor is. Is the same thing as the gas discharge tube that the Hams refer to?

 

[JohnDistai]

So definitely some improvement on the air band. Is there a weak signal or a system you currently can't hear on the higher bands? I guess it's a question of what are you currently receiving well and what do you need to receive better? This discone is a great wide band antenna but the compromise is low gain across that band.

The next city over has some fairly seedy areas, with something headline-worthy each week. I'm trying to stretch reception to catch the traffic in that area.

 

[AB5ID]

The next city over has some fairly seedy areas, with something headline-worthy each week. I'm trying to stretch reception to catch the traffic in that area.

Awesome, what frequency band do they use, and how far away are they?

 

[JohnDistai]

In the 850's. I just checked the map. First, I'm programming the wrong site. Oops! According to the map, the signal range radius ends 2 blocks short of where I am (lol). But I should still be able to get it.

(Edited) Try as I might, it's still too far out.
Oh well. There's plenty of uncivilized folks locally to keep things interesting for now.

 

[mmckenna]

Perhaps I am misunderstanding something on the ground. Any of the grounding-related information seems aimed at Ham sites, so I figured that included scanners. They're talking about perimeter grounds, single point ground panels, common-grounded power at the same spot, etc.

Commercial, ham, scanner or CB, it's all the same. It does need to be grounded, but there is no reason at all why the coax need to run down to the foundation of the house and then back up to the radio. The goal is to provide a nice low resistance to ground. That can be done with #10 or larger wire.

Run that #10 or larger wire down from the lightning suppressor where your coax enters the house to the ground. If there is an existing ground rod there, use that. If not, drive a new ground rod and bond that to the existing home ground point.

I'm not sure I understand what a lightning suppressor is. Is the same thing as the gas discharge tube that the Hams refer to?

Gas discharge tubes are one form of a lightning suppressor/protector. The goal is to give that unwanted energy a way to get to ground easier than running through your radio. National Electric Code requires one where the coaxial cable enters the home. Those need to be provided with a path to ground.

Again, no need to run your coax all the way down to the house foundation. Easier/cheaper to run a ground conductor.

 

[JohnDistai]

Commercial, ham, scanner or CB, it's all the same. It does need to be grounded, but there is no reason at all why the coax need to run down to the foundation of the house and then back up to the radio. The goal is to provide a nice low resistance to ground. That can be done with #10 or larger wire.

Run that #10 or larger wire down from the lightning suppressor where your coax enters the house to the ground. If there is an existing ground rod there, use that. If not, drive a new ground rod and bond that to the existing home ground point.



Gas discharge tubes are one form of a lightning suppressor/protector. The goal is to give that unwanted energy a way to get to ground easier than running through your radio. National Electric Code requires one where the coaxial cable enters the home. Those need to be provided with a path to ground.

Again, no need to run your coax all the way down to the house foundation. Easier/cheaper to run a ground conductor.

Okay, thanks. Perhaps I'm still getting confused on a maintenance practicality issue. If I'm in a 3rd floor attic, and my coax enters there, then my spark gap arrestor would be located OUTSIDE at that entrance point. If I'm understanding correctly, I'd need a ladder that tall to check and replace the gas discharge tube. I don't have that. If it were inside, I could use an attic scuttle way to change it, but my understanding is that all ground-related connections occur outside. Am I understanding this correctly?

 

[KC3ECJ]

Perhaps I am misunderstanding something on the ground. Any of the grounding-related information seems aimed at Ham sites, so I figured that included scanners. They're talking about perimeter grounds, single point ground panels, common-grounded power at the same spot, etc.

I'm not sure I understand what a lightning suppressor is. Is the same thing as the gas discharge tube that the Hams refer to?

Some arrestors/suppressors are more complex than others.

 

[mmckenna]

Okay, thanks. Perhaps I'm still getting confused on a maintenance practicality issue. If I'm in a 3rd floor attic, and my coax enters there, then my spark gap arrestor would be located OUTSIDE at that entrance point. If I'm understanding correctly, I'd need a ladder that tall to check and replace the gas discharge tube. I don't have that. If it were inside, I could use an attic scuttle way to change it, but my understanding is that all ground-related connections occur outside. Am I understanding this correctly?

For what you are doing, you could install it inside and be OK.

The idea is to do the best you can with your particular installation. Your system will not survive a direct strike, so don't get hung up on that. The code is to help reduce risk for induced energy from nearby strikes. Put the protector right inside, and give it a nice path to ground.

Trust me, you are miles ahead of what most hobbyists do. Most don't install any grounding or protection.

 

[KC3ECJ]

Put the arrestor directly on the antenna or on a short piece of coax before the antenna, it has a grounding wire from the arrestor going directly to a rod, that rod also tied into the common ground such as the freshwater pipe to the street, or the main breaker panel and it's rod?

 

[tvengr]

The greatest height I could indoors required a 50ft cable run. I bought a 75' cable just to be safe.

If you only need 50' of cable, don't use 75'. Longer cable = more loss. Both RF and $$$$.

 

[paulears]

In the UK, despite our sometimes nit picky rules, we do NOT, as a standard, fit any form of lighting protection here. The vast majority of non-commercial radio installations of all types run the antennas to the equipment. A few folk (often with access to the protection via their work) might fit protection, but it's really not common. Even worse - our electrical installation standards are very keen that extra earths (grounds) are not fitted willy-nilly. Our electrical installations have a one point grounding system - at the incoming cable and fusing where the house electrical grounds are connected to neutral. Things like out buildings can have their own electrical ground if needed, but then that ground is totally separate from the house ground - no common paths. At commercial radio sites, grounding is of course quite different with everything tied to a good ground with usually gas discharge protection.

In homes, we don't usually do anything with the cables from radio or TV/Sat antennas apart from connect to the equipment. We do get lightning but it's not really that much a problem that we change our rules for it. New electrical installs in domestic property often now feature surge protection devices - but if you want an antenna for your scanner, we'd just connect to the equipment with the shortest cable you can manage for the lowest loss.