adding a bleed resistor to my atu against static build up - Page 1
Forum section: electronics / rf-microwave

author: p.larner
author_rank: Frequent Contributor
timestamp: March 05, 2024, 08:48:34 pm
content: if i add a resistor between my 1-1 atu balun output and ground to bleed off antenna static will it screw up the matching?

author: Roger Need
author_rank: Regular Contributor
timestamp: March 06, 2024, 01:09:00 am
content: Quote from: p.larner on March 05, 2024, 08:48:34 pmUse a 100K for the static bleed and you are good to go with no effect on tuning. If you are running 1000 Watts transmitter power you will need 1/2 watt resistor.Roger

author: coppercone2
author_rank: Super Contributor
timestamp: March 07, 2024, 02:58:54 am
content: I wonder what the best resistor is for this. First I thought maybe composition ceramic because its more ideal. But on the other hand maybe wire wound is better because it forms a inductor that ends up having a smaller effect on the antenna? I think the main problem is resistor capacitance. I think it should ideally behave as a choke. Also wonder if a resistor was giving a problem, could you slip a ferrite bead on it ?But I think you want a beefy resistor there, one of those, because it should be able to survive some atmospheric activity (cold, wind, ice, possibly some lightning). It might be hard to verify the integrity of said resistor if its on some antenna up on a pole. I think if you could one of the beefy vitrified power resistors with thick (or possibly welded) leads.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: March 08, 2024, 10:51:21 am
content: Use high Voltage resistors, typically like this eBay item: 302811692019I use them in my Palstar AT1500CV, I don't know why they aren't included as standard.SJ

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 08, 2024, 11:19:12 am
content: Remember, the two factors controlling resistor selection are what is the highest RF voltage expected across it (under normal operation), and how much power might it dissipate (also under normal operation). You may want to consider going all the way, and include a GDT in the design. A few years back NY4Q had a short post on QRZ about lightning suppressor for open feedline that was for sale. I'm not able to find it now, but had a brief circuit design and Mouser part numbers. Other than the mechanical parts, it required two GDT devices, two HV caps, and two 1-meg 1-kV bleed resistors. Looked to be fairly simple to build. Some of the HV resistors were unobtanium during the last several years, but I have secured a few for my needs/projects.

author: CaptDon
author_rank: Super Contributor
timestamp: March 08, 2024, 02:52:57 pm
content: Using a typical 1/2 watt resistor is a bad idea due to nearby lightning strikes and the EMP induced voltage from the balanced antenna (which in the case of lightning is just one big piece of metal in the air) to earth ground. More likely a 10 watt High Voltage resistor is the norm here. Voltages can easily be induced during a thunderstorm that would easily jump across the leads of a typical carbon 1/2, 1 or even 2 watt resistor. The bleeders at our A.M. broadcast directional array were about 10 inches long bridged by open air spark gaps.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: March 08, 2024, 02:59:36 pm
content: CaptDon, don’t forget the OP is looking to add them to his ATU, which has relatively close spaced variable capacitors that will arc over before the resistor arcs.SJ

author: CaptDon
author_rank: Super Contributor
timestamp: March 08, 2024, 03:08:57 pm
content: Agreed, I hate to see those arc 'pit marks' on tuning caps!! Had a lot of them when using under-rated plate and loading vari-caps on my ham radio home brew amps!! It is amazing how those Hi-Q resonant loop (MFJ Super Loop for example) antennas can build around 10KV of resonant voltage across the gap tuning cap from only 100 watts of R.F.!!

author: A.Z.
author_rank: Super Contributor
timestamp: March 08, 2024, 07:52:29 pm
content: Quote from: cosmicray on March 08, 2024, 11:19:12 amit isn't just the power, it's the reflected one, he's going to add the bleeder at the input of his ATU, mind that

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 08, 2024, 09:34:18 pm
content: Quote from: A.Z. on March 08, 2024, 07:52:29 pmThe intention of the bleeder resistor is to drain off static buildup. If the antenna system has reflected power, that is a whole different subject. As to where the bleed resistor is connected, it must have one side connected to earth ground. The circuit I'm most familiar with has the bleed resistor in parallel with the GDT, in a surge protection box.