I have been using a par end fed for my R75 for over a year now and have been quite impressed with it performance using different wire configurations. The weather warmed up today so I decided to build a dipole for it. I used a Jetstream center insulator from universal and for now 50' of 16Ga stranded copper on each leg with a dogbone at each end and some dacron between trees. I fed it to the radio with some quad shield solid copper RG6. The Par was great, but the dipole lowered my noise floor and raised my signal quite a bit. I have to use the ATT. now to listen to New York air control! My question is, it seems with the dipole I should not ground the outer shield from what I read. Is this true and should I just disconnect the coax when not in use?
I built a dipole for my R75
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k9rzz
Member
The shield on any feedline should be grounded, but for receiving ... do whatever works best!
For safety sake, yes, disconnect all your antennas when not in use (or at least when storms are approaching).
For safety sake, yes, disconnect all your antennas when not in use (or at least when storms are approaching).
Ground the shield? Depends on why, and exactly what you mean by 'grounding'.
Grounding the shield of coaxial cable means that 'half' of it is connected to 'ground'. It does absolutely nothing about the 'other half' of it, the center conductor, it's not 'grounded'. So, if you do ground the shield you are 'half' protected.
The only two reasons for 'grounding any antenna is for RF purposes and for safety. A dipole antenna doesn't require/need a ground for RF purposes, it's a 'complete' antenna to start with. sometimes (not often) grounding the shield -may- reduce 'noise' to some extent. Try it and see. If it 'helps' then leave it grounded.
For safety purposes, commonly described as protection against a lightning strike (or near one), sure, it's a good idea to ground both conductors of that cable. It's also a good idea to do that grounding at a point outside of and away from your house. (Tie a string on the end of that cable and drop it out of the window, sound reasonable?)
Safety grounding is NOT as simple as it may sound. It basically means giving current a place to go rather than into or through your equipment and house. A very nice place to start learning about that is with the National Electrical Code (NEC), it gives you some -minimum requirements-. Another source of this sort of information is to read the information supplied by the various people/companies that do that sort of thing for a living.
Have fun.
- 'Doc
Grounding the shield of coaxial cable means that 'half' of it is connected to 'ground'. It does absolutely nothing about the 'other half' of it, the center conductor, it's not 'grounded'. So, if you do ground the shield you are 'half' protected.
The only two reasons for 'grounding any antenna is for RF purposes and for safety. A dipole antenna doesn't require/need a ground for RF purposes, it's a 'complete' antenna to start with. sometimes (not often) grounding the shield -may- reduce 'noise' to some extent. Try it and see. If it 'helps' then leave it grounded.
For safety purposes, commonly described as protection against a lightning strike (or near one), sure, it's a good idea to ground both conductors of that cable. It's also a good idea to do that grounding at a point outside of and away from your house. (Tie a string on the end of that cable and drop it out of the window, sound reasonable?)
Safety grounding is NOT as simple as it may sound. It basically means giving current a place to go rather than into or through your equipment and house. A very nice place to start learning about that is with the National Electrical Code (NEC), it gives you some -minimum requirements-. Another source of this sort of information is to read the information supplied by the various people/companies that do that sort of thing for a living.
Have fun.
- 'Doc
majoco
Stirrer
50feet x2 is about 30 metres, close to the 30 and 31metre broadcast band - so your reception on that band should now be pretty good in the daytime - also after dark you should also get reception around 90metres so the tropical bands should be good. Other harmonics will be good too, so you should have daytime reception on the 19 and 16metre bands. Where your main losses are going to be is up at the join between the dipole and the feeder - you have joined a BALanced antenna to an UNbalanced feeder - you need a BALUN - a 1 to 1 transformer up at the centre point of the antenna. Google for it and you will find many do-it-youself designs, or alternatively just wind a few turns of the coaxial cable a few feet below the antenna centre point - 5 turns will do on about a 6inch circle, just tape them together . A TV antenna balun will not work!
Congrats on the new dipole! Normally ATTenuation is useful under severe overload conditions, like guy-next-door running a kilowatt of power....
The R75 has some very nice preamps that you might want to turn off before applying attenuation. Normally I only ever used Preamp2 for 21 mhz and above, preamp1 for 10-21 mhz, and preamps off under 10 mhz. This isn't set in stone of course, but a general guide.
You can generally get a better signal to noise ratio by disabling preamps first before applying attenuation. The worst case would be having both preamp2 on as well as attenuation enabled!
As LtDoc said a dipole antenna does not require any specific grounding. If you're in a bad weather area then consider buying an arrestor. The arrestor would be at your house feedpoint and connected to earth ground. The R75 should already be grounded to the same common ground.
Thanks guys,sorry I have been gone for a few days. Two new questions though. A 1:1 baulin was mentioned, I have read just as many posts saying it is not needed. Also as Nickcarr said I would like to ground the R75 itself. Should I just take a wire to a chassis screw and run it to my central ground?
Disregard my comment about grounding the R75. I thought it had a ground point.
I looked up the Jetstream insulator. Are you using RG6 coax? Ideally, you'd want RG8 at the very least.
Then you could just buy a Polyphaser unit. Mount the the Polyphaser to a ground rod. Then you're protected against static and (in theory) lightning.
Here's a PDF on how this is used: http://static.dxengineering.com/global/images/instructions/Lightning Protection P2.pdf
If cost is an issue then just disconnect your antenna manually each time a storm comes in...
k9rzz
Member
If I may ask a question since this is on the subject I was also curious about.
The R75 that I just bought and have not received yet has both SO239 and wire antenna connections. If I use a coax to exit my house before connecting to a wire antenna, it's not necessary to connect the shield of the coax to ground? There is only one ground terminal on the back of an R75 receiver and that is the negative side for a wire antenna. If I chose to use a wire antenna from the receiver, am I to understand that I really only need to run one wire from the plus side to the outside wire antenna?
I understand how a system ground works with a tx rig but am I also to understand that a ground on a receiver is only for safety and has no benefit to receiving?
The R75 that I just bought and have not received yet has both SO239 and wire antenna connections. If I use a coax to exit my house before connecting to a wire antenna, it's not necessary to connect the shield of the coax to ground? There is only one ground terminal on the back of an R75 receiver and that is the negative side for a wire antenna. If I chose to use a wire antenna from the receiver, am I to understand that I really only need to run one wire from the plus side to the outside wire antenna?
I understand how a system ground works with a tx rig but am I also to understand that a ground on a receiver is only for safety and has no benefit to receiving?
ka3jjz
Wiki Admin Emeritus
It is certainly possible that a good RF ground (and I am being very specific here) can help lower your overall noise floor. A good station ground will help bleed off static charges, and may mitigate (to some degree) the effects of a strike or near-strike. How much it will help depends on so many factors, that whole books have been written on the subject.
Mike
Mike
Well, I'd say you need a refresher course then. Like Mike said, a quality ground system is a good thing.
Hint: the receiver has a ground connection for a reason.
I suggest reading W8JI's excellent site for more information:
Receiving_basics
Ground Systems
Home page: Antennas Radios Amplifiers Baluns and Receiving Systems for HF
Well, I'd say you need a refresher course then. Like Mike said, a quality ground system is a good thing.
Hint: the receiver has a ground connection for a reason.
I suggest reading W8JI's excellent site for more information:
Receiving_basics
Ground Systems
Home page: Antennas Radios Amplifiers Baluns and Receiving Systems for HF
What I was talking about in reference to the ground on the R75 receiver was that the only ground terminal was that of the negative side of the wire antenna. There is no other ground terminal like my R7100 has.
I understand that a grounded receiver is important. It was only the discussion I have read that sounded like a debate about grounding the shield of a coax feed that confussed me.
I've got at least three 8' long ground rod in the ground in my back yard. I use to run a Jo Gunn 4 element verticle and horizontal beam and a omni directional Spiro ground plane antenna.
Thanks for the information.
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