G5rv configuration and performance problem

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE0BXN, Mar 9, 2018.

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  1. WA7ARK

    WA7ARK Ham Member QRZ Page

    Next time you disconnect the PL259 from your rig, short the center pin to the shield with a paper clip, clip lead, or even a 1MegOhm resistor. No more little arcs.
     
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    3V:

    I don't know about where you live in Pennsylvania, but here, in Texas, when the wind "picks up", especially on wire antennas and other types of antennas that do not have a DC grounded feedpoint, the Van der Graff effect definitely builds a voltage to where it definitely can be high enough to arc across a PL-259.

    When the dust blows in from west Texas, the generated voltage increases even more.

    On tube type equipment, very seldom is there any damage from this voltage. But, on some solid-state equipment, there definitely can be damage. Putting a fairly high value resistor across the coaxial cable will "bleed off" this voltage and prevent damage to the equipment.

    It is those people who think that disconnecting the cable will prevent any damage who "discover" the fact that there is a high voltage being generated across their antenna(s). As for lightning protection, disconnecting the cable can actually increase the possibility of damage from a nearby or direct strike. Frankly, there is a very good reason that the shield, of any coaxial cable, be grounded at the point where the cable enters the building and that is to dissipate any charge to ground before it can enter the building.

    Right now, the wind is gusting to almost 30 mph and that is pretty much a "normal" day. Much higher speed gusts are not uncommon and, especially just before a storm front moves through, winds can reach hurricane speeds and, in some cases, a fair amount higher. I have seen gusts to 100 mph over the years and gusts to over 70 mph happen several times a year. Of course, living 1/2-block from the highest point in the city does contribute to having the higher velocity winds.

    Glen, K9STH
     
  3. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    I meant to credit Cecil, W5DXP, for the chart I used in my post #16. It was very good of Cecil to make those calculations and post them graphically for us. This is a very useful graphic which demonstrates a number of points about what we can reasonably expect from the G5RV (as well as the ZS6BKW).
     
    KC3BZJ likes this.
  4. KE0BXN

    KE0BXN Ham Member QRZ Page

    As promised, here are a few of the smith charts and SWR from 3-30 from the Analyzer.

    [​IMG][​IMG][​IMG][​IMG]
     
  5. AI3V

    AI3V Ham Member QRZ Page

    The national weather service tells me your average wind speed is less than 10 miles per hour.....

    Like I said, videos or it didn't happen. :)

    Rege
     
  6. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Well, the Smith charts seem to be way off, particularly the 15m one.

    As for the SWR curve, it looks like it might be close to what we might expect, with dips that might (or might not) be for 75, 40m, and 20m. I can't be sure, though, because there are only four labeled frequency divisions between 3 and 30 MHz, and those have oddball values instead of easily interpreted ones or specific markers for each of the ham bands.

    I know how to extrapolate the data provided and come up with exaclty where 7, 14, 21 MHz, etc., etc., would be on the chart, but I'm afraid I don't have time to do it. Perhaps others, more used to app-driven analyzers, can interpret this curve more readily.

    In addition, there seems to be an unexplained dip somewhere close to where 17m might be; and another near where 12m might be. Once again, are we certain this is a conventional G5RV, or is it a variation?

    Were I in your shoes, I would definitely employ whatever method I have handy and verify those analyzer SWR measurements. In most cases, the quickest and easiest way to do this is to use the on-board SWR meter most modern rigs have, and take SWR measurements across the amateur bands of interest, taking not just one reading per band, but making a sweep of the band, like the analyzer does.

    Almost all who build antennas (and I can build only very simple ones) make SWR curves when they first put the antenna up, to give a baseline of readings, which in turn will often make diagnosing future problems easier.
     
    Last edited: Mar 10, 2018
  7. G0JUR

    G0JUR Ham Member QRZ Page

    Like this?

    or this

    and definately this


    that enuf for you?
     
  8. W5DXP

    W5DXP Ham Member QRZ Page

    What kind of coax are you using?

    What would be interesting if TLDetails were used with the measured shack impedances to see if they correlate to the earlier Smith Charts which do not include the coax.
     
  9. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    3V:

    There is a BIG difference between "average" wind speed and what speed wind "gusts" can achieve as well as speeds when weather fronts come through, during storms, and so forth.

    At this particular moment, the winds are calm which means, basically, a "0" wind speed. That may happen for anywhere between several minutes and several hours. Then, the wind can "pick up" and blow for like 8 or 10 mph for between several minutes and several hours. However, there can be wind gusts well over 30 mph during that period. There are times, especially during the spring and summer when the wind can be sustained at well over 20 mph, even over 30 mph. But, in general, even those winds only last for several hours at most.

    When cold fronts come through, when warm fronts come through, and especially when a "storm" front comes through, that is when the wind speed increases and gusts to 60 mph to 70 mph happen a number of times during the year and there have been times when the wind gusts have gone over 90 mph and even have broken the 100 mph barrier.

    But, taking the "average" of all the wind speeds, yes, that comes out to 10 mph or less.

    Here might be the wind speeds for a 24-hour period: 8 mph for 225 minutes (3.75 hours), 12 mph for 90 minutes (1.5 hours), 3 mph for 510 minutes (8.5 hours), 0 mph for 60 minutes (1 hour), 30 mph for 5 minutes (0.0833 hours), 45 mph for 5 minutes (0.0833 hours), 27 mph for 5 minutes (0.0833 hours), and 7 mph for 525 minutes (8.75 hours).

    Calculating the average:

    8 x 225 = 1,800

    12 x 90 = 1,080

    3 x 510 = 1,530

    0 x 60 = 0

    30 x 5 = 150

    45 x 5 = 225

    27 x 5 = 135

    7 x 525 = 3,675

    Total is 8,595

    Divide that by 1,440 (60 x 24) and you get 5.96875 or about an "average" wind speed of 6 mph. However, during that 24 hours there were wind speeds of 45 mph.

    Of course, these figures were "made up", but they show how an "average" figure just does not really have anything to do with "top" speeds during the period. Also, since there are 365 days in a year, there definitely can be a significant number of days that have wind speeds even over 30 mph and yet when the "average" is calculated, because there can also be days with very low wind speeds, that "average" can definitely be below 10 mph.

    You do not live in this area and, as such, you really do not have a "clue" as to what things are like. I suggest that you move to the south-west, or some other area in which the humidity can, at times, be low and where the wind speeds can, at times, be relatively high. Then, I think that you just might form a different opinion on just how much static electricity can "build up" and whether or not that level is sufficient to "jump over" from the center pin of a PL-259 to the shell.

    Glen, K9STH
     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    W8JI had the same "I'll believe it when I see it with my own eyes" attitude because he lives in GA. I personally have not seen the problem here in East Texas where presumably the humidity acts like a bleeder resister to discharge the P-static without arcing at the PL-239. But I lived for 10 years in Queen Creek, AZ and have heard P-static discharge sounding like a machine gun with 5-10 discharges per second during a heavy dust storm. Someone reported being able to hear the collision of large individual snowflakes on his receiver while watching his antenna during a snowstorm. It's been reported to happen just as the first drops of rainfall start falling during a rain storm. It is extremely common in moving aircraft. The only difference between a moving aircraft and a ham antenna is that the aircraft is moving faster than the air and the ham antenna is standing still while the air is moving fast. In both cases it is relative motion that matters.
     
    Last edited: Mar 10, 2018

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