HFU HF Underground
Technical Topics => Equipment => Topic started by: flexoman61 on June 11, 2016, 1525 UTC
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....is it necessary? My receiver is grounded, about 6' from ground rod, and coax is connected to
the receiver by a PL259 connecter. Isn't the braid already grounded by the receiver?
This is just a little confusing with a folded dipole. Isn't the center conductor and the
braid in continuity within the antenna? Seems if the braid was grounded you would lose
some signal to ground with a folded dipole. I'm having no noise or signal problems with
my folded dipole, just trying to understand the theory. I hope this makes sense!
Thanks for any input.
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You should have a balun at the feedpoint of the antenna to ensure both wings of the antenna are balanced, and see the correct load impedance. A folded dipole of the correct dimension is around 200-300 ohms balanced. A common tactic on VHF and UHF frequencies is to short one end of the element to the midpoint, and take the coaxial feed to the remaining feedpoint and the midpoint of the antenna. At this point, the impedance should be between 50-75 ohms, but the antenna may suffer some imbalance.
As for grounding, you DO NOT want strike current passing though your equipment. The only correct way to do this is to bring your coax line to a ground block that is directly tied to station ground, hopefully directly to the ground rod. This way, all high current will go directly to ground and avoid passing through your equipment, causing further damage. In radio and TV I've seen a lot of damage over the years caused by poor grounding practices.
+-RH
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Yes, a folded dipole usually has both legs at dc ground, but if the only ground is provided by the coax attached to your radio, everything is flowing to your radio, signals, static, lightning discharge, etc. So, you might want to make a grounded bulkhead panel and place a surge suppressor there, feeding the rig with a short piece of coax. I like these suppressors;
http://www.polyphaser.com/products/rf-surge-protection
But there are cheaper ones out there that are perfectly suited to the hobbyist such as these;
http://www.aesham.com/product-category/antenna_accessories/antenna_accessories_lightning/
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I've built several folded dipoles over the years, I presently have one for 48 meters for Europirates, and am considering making one for 4 MHz for the SW USA pirate beacons, which may also serve double duty for the 90 and 60 meter SWBC bands. I always use a 4:1 balun, and feed with RG6 coax, which is 75 ohm. Seems to work pretty well. I like using RG6 because it is good, cheap, easy to find, and already has molded F connectors on each end. I just use an F to UHF adapter at each end as necessary. You can wrap that up with coax seal if you wish.
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I'm using a 4:1 balun with RG-6 coax.
I will install a grounding block and surge suppressor.
The coax run from ground block to receiver will be 50',
hopefully that's not too long.
Thanks for input.
Chris, do you ground your folded dipole?
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I do where it enters the shack. I also make it a habit to disconnect all of my antennas from the radios when storms are likely, for extra safety.
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You might also look into some ferrites around the coax at each end, or just at the shack end. These help isolate the coax from the antenna, helping in balance of the pattern and keeping noise from flowing in or out of the shack. I use a few at each end, but testing has shown their actually most effective at the shack end for some reason.
http://www.allelectronics.com/make-a-store/category/235/ferrites/1.html