Grounding: bonding dissimilar metals & phasing two 4 bay TV antennas - Page 1
Forum section: electronics / rf-microwave

author: videobruce
author_rank: Frequent Contributor
timestamp: November 12, 2016, 05:31:50 pm
content: Two questions:1. Two 4 bay TV antennas, same array, I know the length of each cable to the combiner (splitter) should the the same length, but does it matter how long those cables are? IOW's to place the combiner in an enclosure well below the antennas for easier access and weather protection, is there any problems with extending the length another 5-10'?2. Using 1 1/2" EMT conduit for a mast, I need to bond a double run of #12 copper ground wire to it. Six gauge isn't available. Being dissimilar metals, is that a problem?Hope all of that made sense.

author: cvanc
author_rank: Frequent Contributor
timestamp: November 12, 2016, 09:53:15 pm
content: A mast made of 3/4" EMT? I hope you live in a very low wind area. Strong risk of it folding over otherwise. I'd hate to see all your work take a spill...Maybe consider a larger diameter iron plumbing pipe instead? Good luck.

author: SeanB
author_rank: Super Contributor
timestamp: November 13, 2016, 07:21:54 am
content: Grounding connection is easy, use a grounding clamp to attach the wire, and coat liberally with dielectric grease, or even ordinary wheel bearing grease, to at least change the corrosion type. It will still corrode, but this will be a lot slower than electrolytic corrosion of the wire.

author: German_EE
author_rank: Super Contributor
timestamp: November 13, 2016, 09:35:26 am
content: I would connect the copper ground wire to a stainless steel clamp then fit the clamp to the mast, making sure the copper wire did not touch the aluminum.

author: videobruce
author_rank: Frequent Contributor
timestamp: November 13, 2016, 01:58:51 pm
content: Correction, the mast is 1 1/2", the brackets are labeled 3/4" IP. My bad.I do have a grounding clamp for a mast that will be used so copper to EMT won't be a problem. I do use clear spray over all hardware (bolts, screws, nuts etc.)

author: SeanB
author_rank: Super Contributor
timestamp: November 13, 2016, 06:44:26 pm
content: Not clear spray, it cracks with exposure to sun and with UV exposure. A good product is Tectyl 506 spray, it works really well, and will last for decades in direct sun with no hassle and only dust build up. I have undone bolts exposed to salt spray ( I work near the sea) for decades, that were coated with this, and they undid easily ( after the initial brute force to start them) and the threads were nearly perfect under the coat, and the surfaces were unpitted, even dissimilar connections.BTW be very careful using stainless steel hardware on aluminium parts, the stainless will not corrode, but the aluminium will corrode right next to the stainless, and this intergranular corrosion will be only visible as a white haze on the surface, until the part fails from being corroded all the way through. You need to use a Monel metal fastener, it will not react with the aluminium, and is used in shipbuilding to attach hardware to yacht masts, rigging and engines, but they are pretty expensive. otherwise dielectric grease, placed under the parts as well as over, will slow it down considerably.

author: videobruce
author_rank: Frequent Contributor
timestamp: November 14, 2016, 04:26:09 pm
content: Can phase cancellation have a factor explaining the following variances of signal level? I do have a general idea of phase cancellation, but I just ran into a scenario that I never did before.The antennas are 4 bay, bow tie UHF TV antennas. There are a number of similar designs sold under different names, some grossly overpriced marketed to a limited knowledge consumer.These are sold either as a single 4 bay or a dual 4 bay (8 bay total) with a wiring harness comprising of a common 'splitter' (combiner) and a pair of RG59 'jumpers.All appear to be based on what I call the 'gold standard' of UHF TV antennas, the original US made Channel Master 4228(A). There have been plenty of articles and tests on this model in the past. Problem is PCI International (that makes lousy 'F' fittings) bought CM many years ago, closed the plant in the south and farming out a much cheaper design from China (nothing unusual). Worse yet,, they conveniently gave it the SAME model number, adding a 'HD" at the end (4228HD). Typical marketing scam.Anyway, what I encountered was performance differences from a single 4 bay to the 8 bay model on the same mast. The 4 bay above the 8 bay pointed in the same direction, measuring the same station 60 miles away within seconds of one another w/ a portable SLM/spectrum monitor (much older Tek SignalScout). The single bay outperformed the 8 bay. Testing each of the dual 4 bay's on the 8 bay array by themselves gave mixed results. One single antenna had no signal, the other had some, but not as much as the top single way bay.The single antenna is 20"x32". Horizontal centers would be 22" on the 8 bay, vertical center would be 35" between the single & the dual antenna. The frequency of test was 671 MHz (ch. 47) and the distance was 61 miles, 1/3 over water, 1/3 over land below an escarpment and the rest fairly level ground with many tress nearby (in the path w/o leaves).I did NOT use the supplied RG59 32" jumpers. I used two RG6 8' (approximate) jumpers to place the combiner in a weatherproof enclosure that would be easily reachable.Specific antennas are here;www.mcmelectronics.com/product/STELLAR-LABS-30-2430-/30-2430www.mcmelectronics.com/product/STELLAR-LABS-30-2425-/30-2425