advertisement


Outside FM Antenna grounding

Gervais Cote

Gervais Cote

Predator
Gents,

I just finished the install of an fm antenna and I am wandering if the in ground earthing is okay.

The set up is that I have a coaxial cable from the antenna connected to a junction/grounding block and then another coax cable to my Naim NAT01 tuner.

The junction block is bolted to a galvanized steel structure that was previously used for a large tv antenna so the grounding is actually done by the metal to steel contact between the junction block and the galvanized steel structure. This galvanized steel structure is 4 feet deep in the ground. The junction block has a green screw that’s designed to tighten a 10 gage copper cable to allow grounding with a copper pin in the ground.

So my question is : is my actual steel on steel grounding enough against thunder strikes or should I add a 10 gage copper wire and a 3 or 4 foot long copper pin in the ground ?
 
Just make sure that there is electrical continuity between the down cable ground and the galvanized steel structure, using a multi-meter, and all should be good to go.
 
Craig B said:
Just make sure that there is electrical continuity between the down cable ground and the galvanized steel structure, using a multi-meter, and all should be good to go.
Click to expand...

That could be bad for signal integrity though. Depending on the design the dipole elements are often insulated from the rest of the array. This "junction box" may have spark gaps etc to deal with lightning but not ground the coax directly.
 
When I had Ron Smith and other aerials for my 01, the coax was simply connected to the box on the array. Filled one with wax once. As it was on a 12' mast above chimneys I guess it wasn't grounded for lightning strikes, not that I ever had one.
 
This is somewhere where cables do make a difference BTW! Use top quality thick low loss coax if going any distance as you will lose half your signal in the cable with cheap stuff...
 
I connected the coax cable to my Kenwood KT 8300 tuner and so far, here are the results :

-The vu-meter that indicates the signal strength went from 4.5 to a perfect 5 on a scale from 0 to 5 so this shows the signal is better with my aerial than it was with my previous flat tape antenna.

-There is a pink noise in the background now when I listen to fm radio (a continuous shhhhhh) when my selector is in stereo mode. The fm sound becomes dead quiet without any pink noise when I switch from stereo to mono.

-And since I am using the aerial instead of the flat tap antenna, the resolution of the treble has improved, I have no more sibiliance at all.

According to these results :

-Should I isolate the transfer block from the galvanized steel structure ?

-And if yes, should I ground this transfer block with a 10 gage copper wire and an in ground copper rod ?

By the way, I am using the best quality of coax cable I was able to find at my regular electronic store, it’s a black one designed to be used outside and inside. The first coax cable is 15 foot long and the second one is 35 foot long.
 
Gervais Cote said:
I connected the coax cable to my Kenwood KT 8300 tuner and so far, here are the results :

-The vu-meter that indicates the signal strength went from 4.5 to a perfect 5 on a scale from 0 to 5 so this shows the signal is better with my aerial than it was with my previous flat tape antenna.

-There is a pink noise in the background now when I listen to fm radio (a continuous shhhhhh) when my selector is in stereo mode. The fm sound becomes dead quiet without any pink noise when I switch from stereo to mono.

-And since I am using the aerial instead of the flat tap antenna, the resolution of the treble has improved, I have no more sibiliance at all.

According to these results :

-Should I isolate the transfer block from the galvanized steel structure ?

-And if yes, should I ground this transfer block with a 10 gage copper wire and an in ground copper rod ?

By the way, I am using the best quality of coax cable I was able to find at my regular electronic store, it’s a black one designed to be used outside and inside. The first coax cable is 15 foot long and the second one is 35 foot long.
Click to expand...

if you have total 50 foot of coax then you really need good quality stuff. You could be losing easily half of the signal from the aerial and maybe more if not using low loss coax. Specs are usually given as per 100 foot and at that frequency would vary from as much as 10dB to as low as maybe 1.5dB. The stuff that gets as low as 1.5dB is pretty serious though.... as in like 3/4" thick. Some "generic brands" may be worse than 10dB.... 3dB loss means you've lost half of the signal from your big expensive aerial.
You know how you get mast head amplifiers? Ever thought it a bit weird to go to the lengths of putting it up there and running power to it etc when you could put it at the TV/radio end? This is because there is so much loss in the cable itself and it you get a much better signal to noise ratio if you boost the signal before then attenuating it through the cable;)
 
It is much better to use good low loss coax than to use a mast head booster. The latter can cause extra "noise" due to intermodulation.
If your aerial is likely to get a direct strike, you need both a lightning surge arrester on the coax and a lightning conductor above the aerial to take the hit. The arrester protects against induced surges, nothing electronic can survive a direct strike
 
Craig B said:
It was all twisted 300 Ohm spaced flat line back in my day!
Click to expand...

Was that the Middle Ages, Craig? :). My first serious (8 element Antiference) antenna went up in 1970 and standard 75 ohm coax was the only relevant cable then (and before). You must mean the internal throw-away stuff sometimes supplied with Japanese tuners back in the day.

O.P. Can't understand why you don't simply connect the array to your 01 directly; one uninterrupted coax. I've never experienced an antenna connected in any other way.
 
Last edited:
I'm a little concerned that you're exporting an outside earth into the property through the co-ax screen. I don't think this will be an issue with the NAT01 because the tuner head input is effectively the floating primary winding of an RF transformer in the can. The coax is AC coupled to the case however through a very small (pF) capacitor. The case is 0v for audio that may connect to mains earth via the pre-amp and any grounded equipment connected to it. Technically, and especially if your property has PME earthing, then the coax and tuner could theoretically be at different potentials. I think ideally you'd want a surge arrestor on the co-ax screen to ground, but it shouldn't be directly connected to it. I assume the antenna is a folded dipole with a balun on it.

What everyone says about low loss coax is true. I don't think you can get the really awful, almost non-screened, shitty brown stuff any more. A decent satellite rated cable will be good for broadcast VHF frequencies.
 
Mike Reed said:
Was that the Middle Ages, Craig? :). My first serious (8 element Antiference) antenna went up in 1970 and standard 75 ohm coax was the only relevant cable then (and before). You must mean the internal throw-away stuff sometimes supplied with Japanese tuners back in the day.
Click to expand...
Nope, 300 Ohm twin lead from stand alone tower mounted yagi to basement recreation/tv room. The tower went up in the summer of 1966. The nearest TV broadcasters were 65 miles away and the general consensus at the time was that, as long as care was taken wrt routing (long stand-offs with plastic insulators, twisted along length to reduce RFI susceptibility, etc.), signal loss was thought to be roughly half that of co-ax. In practice, it likely wasn't as good as advertised, however, we kids still enjoyed our Saturday morning cartoons, and the Ed Sullivan show never looked better.
 
Craig B said:
however, we kids still enjoyed our Saturday morning cartoons, and the Ed Sullivan show never looked better.
Click to expand...

Sorry, Craig. Guess you were talking about TV (425 line then?). FM always used coax as far as I can remember to the mid sixties. American TV prog. on British TV then? I'm surprised, not that I ever watched TV in the swinging sixties in Hampstead; at least, sober or unstoned.
 
Mike Reed said:
Sorry, Craig. Guess you were talking about TV (425 line then?). FM always used coax as far as I can remember to the mid sixties. American TV prog. on British TV then? I'm surprised, not that I ever watched TV in the swinging sixties in Hampstead; at least, sober or unstoned.
Click to expand...
I'm in Canada, Mike. After having moved to the mainland (Canada, really) from NF in '65, I lived in a small city 65 miles from Detroit, MI. Some had rotators fitted to their antennae, ours was permanently fixed on The Motor City.

Come to think of it, I never got stoned until I began listening to Arthur Penhallow on WRIF!
 
Craig B said:
I'm in Canada, Mike.
Click to expand...

D'you know, Craig, much as I admire the improvements wrought by the 'new' pfm format, I always find it a pity/nuisance that I rarely know the county, let alone the country, of members who post; info. which is available on much older forums. Especially of help in the classifieds, of course. This is a perfect example of a signal failure of aerial geography.:) Almost an elemental balons-up, as it were.
 
You must log in or register to reply here.


advertisement


Back
Top