Line loss greater than antenna gain?

Status
Not open for further replies.
Joined
Feb 25, 2010
Messages
45
Location
Arapahoe county, Colorado
Here's the predicament, and hoping the good folks here can help me decide on what's best.

Getting a scanner and already I've been on the roof to look at mounting a discone antenna. Fun.

So the antenna is claiming a 6dbi gain. Fair enough. However, with the coax I'm using and length, the run of cable would constitute an 8dbi loss at 800mhz. Not being very educated on this and just looking at the numbers, would it be self-defeating to use the discone under these circumstances?

If the loss I get from the line is greater than the gain achieved by using the antenna, then wouldn't a rubber duckie antenna perform better, or am I failing to account for other factors here?

Thanks in advance.
 

SkipSanders

Silent Key
Joined
Dec 19, 2002
Messages
1,061
You're failing to account for height.

Higher is better, for antennas. Also for attenuation of the signals getting into your house, which depends on your home's construction.

However, another answer might be better cable.

Some might try an inline preamplifier at the antenna, but that can give you overload problems very easily, as they're usually 20db, rather than the 10db that'd be closer for your needs.

Actually, many folks find the bare discone, outside, up high, gives them problems with receiver desensing, due to strong local FM Broadcast signals, or nearby Cellular transmitters, even without amps.

Better cable, and if you're only interested in 800, an 800 MHz gain antenna, instead of the zero gain discone, might work better.

How long a cable run are you using, and what cable type, anyway?

Most discones, by the way, claim more like 2-3 dbi. They are not gain antennas.
 
Last edited:

photogadam

Member
Joined
Feb 19, 2010
Messages
16
Well you have to think of where to actual antenna is to compare to two. 2dbi loss for the discone or 0dbi gain for the ducky at the same location. The question should be, how many dbi are you loosing being indoors (of wherever) compared to being on the roof. Your house may be costing you more than 2dbi.

Honestly I really can't help you with the antenna/coax thing. Maybe someone else can help you with those numbers.
 
Joined
Feb 25, 2010
Messages
45
Location
Arapahoe county, Colorado
The cable would be LMR-195 (I have 1000ft of it heh) and the run would be about 75 feet. I could mount the antenna on the edge of the house as opposed to the top and run the cable through a window which would get it down to about 20 feet.

I considered better cable. I'd hate to have to do that though because I have all kinds of connectors and a crimper for the 195 already. Trying to make that work but if it doesn't I'll have to get better cable.

edit; OH! Wanted to throw this in the mix. Don't know if it matters, but I have a 300 ft. long wire high impedance antenna in the attic for my shortwave. Don't know how that affects things, but probably need to do some trial and error correction :)
 
N

N_Jay

Guest
For the scanner you would be MUCH better of with RG-6 as an inexpensive replacement.
 

zguy1243

Member
Joined
Apr 24, 2006
Messages
308
Location
Calhoun Georgia
It is a good bit of trouble installing a rooftop antenna and not get a good return from it. Your antenna is more important that radio-plain and simple. If your primary concern is 800Mhz I would go for some RG-11 coax for feedline if running over 50 feet. RG-11 is about the size of LMR 400 and it is cheap and performs very well. A discone is always a great multi-band antenna and it should serve you fine. Remember to weather proof all connections properly and ground the system. Spend a few extra dollars on the antenna, you won't regret it. My thinking is what good is it to have hundreds of dollars in a scanner or receiver and then have a 50 dollar antenna setup...
 

dd364

Member
Joined
Sep 5, 2009
Messages
106
Location
western ny
I think your confusing antenna gain with cable loss.dbi refers to the gain in reference to a isotropic radiator.Cable loss is expressed as db not dbi.Two diferent things.
 
Joined
Feb 25, 2010
Messages
45
Location
Arapahoe county, Colorado
Ah yes, you're right dd364. I'm using db and dbi interchangeably.

I've got a few tricks to try out. I built an amplified antenna kit from Ramsey Electronics as well. Going to do some experimentation to get this worked out. And I have time, don't even have the scanner yet, but will soon... as I decide on which one :p

edit; could be I don't need the antenna haha. On this analog scanner I can get police in another town 40 miles away. But that's if and only if I have it next to the lead wire coming down that goes to a 300ft longwire in the attic for my shortwave. I think it's playing a part.
 
Last edited:
N

N_Jay

Guest
50 vs. 75 Ohms

Receive systems are relatively tolerant of mismatches.

Additionally wide band systems (both radios and antennas) are notorious for not being anywhere near their "typical" impedance specification across any significant frequency range.

The two together make the small difference in impedance between 50 ohm and 75 ohm cable almost completely irrelevant.
 

N0IU

Member
Joined
Sep 26, 2009
Messages
802
Location
Wentzville, Missouri
Hmm, I wonder about installing an 18db TV amplifier in line.
Check the frequency range of the amplifier. While it might help on some frequencies, it might actually reject others.

If you use something for another purpose other than for which it was designed, it is not always reasonable to expect the same results.
 
Joined
Feb 25, 2010
Messages
45
Location
Arapahoe county, Colorado
Yeah it says the range is 54-1000mhz, but "return path" is 5-42 mhz (I'm guessing that's rejected) which would be ok cause I'm not really listening to baby monitors or the old 10 channel phones. Guess I could try it, but I'm open for suggestions. Something with a bnc, N, or SO-239 connection would be better.
 
N

N_Jay

Guest
Ah ok, that's good to know. Hmm, I wonder about installing an 18db TV amplifier in line.
The gain of an amplifier is not near as important as its Noise Figure and 3rd order intercept point.

The amplifier should be as close to the antenna as practical.

Getting positive results from an amplifier on a wide band or multi-band system is usually difficult and often you will actually loose overall sensitivity.
 

kb2vxa

Completely Banned for the Greater Good
Banned
Joined
Mar 22, 2005
Messages
6,131
Location
Point Pleasant Beach, N.J.
"Yeah it says the range is 54-1000mhz, but "return path" is 5-42 mhz (I'm guessing that's rejected) which would be ok cause I'm not really listening to baby monitors or the old 10 channel phones."

By way of explanation that has nothing to do with scanners you're referring to CATV equipment here. (;->) The return path is data returned to the cable system head end by "on demand", and other premium service control devices plus internet uploads if your cable service provider is also an ISP. CATV frequency allocations are divided into bands just like on air services, sub band* (5-50MHz) is used for data transfer. The forward path (54MHz-1GHz) includes the bands used for television channels.

* The reason for it being called sub band is it lies below low band, TV channels 2-6.
 
Joined
Feb 25, 2010
Messages
45
Location
Arapahoe county, Colorado
Hey thanks for that. I'm learning a lot here and glad that you folks are taking your time to educate me on this stuff. I only know enough to get in trouble :)

Think I'm just gonna pass on the amp. Going for the 396xt and will work with the factory antenna and then try other options. When I first started listening to shortwave, I went through 4 different configurations before I found something that was great. Experimentation is half the fun anyway. Thanks again kb2vxa.
 

K7CAR

Member
Joined
Nov 12, 2009
Messages
150
Location
Monroe, WA
You won't need an inline amp. Don't bother, all that will do is amplify the noise. The 195 you have is perfectly fine. That short of a run you won't see a difference on a simple scanner receiver. You don't have any way to measure signal strength, so who cares if it's S-3 or S-5. The minor impedance mismatch does not matter one bit on a scanner.

If you are trying to pull out a super weak signal from 50 miles away that you must receive then go for the best feed line, highest gain antenna as high as you can get it. If you're like most on here in or near a city then you can get by with far less.

Don't get so wrapped up in counting db's. 3 db is half an S unit. 6 db is about how much of a difference an experienced operator can discern. BTW, the isotropic antenna (dbi) is a fictional antenna that only exists in free space. Discone or ground plane antenna doesn't show a gain. Antenna manufactures love to show fictional gain numbers that are mostly bunk.

You have 1,000 ft. of LMR 195. Install multiple antennas and see how they perform. You don't need to spend $$$ on scanner antennas. Put one up on your roof. Another in you backyard. VHF-UHF signals can vary by a huge amount by moving your antenna just several feet. What makes that part of the frequency spectrum so fun to play with. Get your Ham license then you can really have fun.
 
Last edited:

kb2vxa

Completely Banned for the Greater Good
Banned
Joined
Mar 22, 2005
Messages
6,131
Location
Point Pleasant Beach, N.J.
"I only know enough to get in trouble."
That's a good sign.

"Thanks again kb2vxa."
You're welcome, if I can help you get in trouble it warms the cockles of my black heart.
 

hokiewheeler

Member
Joined
Feb 18, 2010
Messages
17
dBi is gain over isotropic, which is a theoretical antenna that radiates and receives uniformly in all directions. Since this doesn't exist in practicality, it's more informative to list dBd, or gain over a half wavelength dipole antenna (you can convert by subtracting 2.15 from dBi). Don't worry too much about the antenna gain figures on the scanner anyway. I've got a discone in the attic and I suffer from bad front end over load in the VHF hi band. It actually got worse when I put low loss 9913 cable on it but my 800 receive got better. I used to run generic radio shack rg 8. If I put my antenna outside, I probably wouldn't lose much on VHF but I bet I'd get a lot better on UHF and 800. If you really want gain, you're going to need an antenna specifically for the band you want to monitor or a directional antenna.
 
Status
Not open for further replies.
Top