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Progress on 160m invL + 80m vertical + !!

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by M0AGP, Sep 18, 2021.

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  1. M0AGP

    M0AGP Ham Member QRZ Page

    Building the new antennas and managed to get a nice shot showing the progress plus the fan dipole in the background (it's rare you can ever see the fan dipole):
    But I spotted something lurking in this photo - here is a zoom-in!
    The 10m wire is broken!!!! But it still works on 10m...
    Ah - the 30m wire is acting as a 3/2 wave!
    Why did I ever bother with the 10m wire??
    I wonder if a falling branch did this.
    Well it comes down tomorrow AM for repair!
    It's a good thing phones have high resolution camera these days...
    G6YYN and US7IGN like this.
  2. M0AGP

    M0AGP Ham Member QRZ Page

    Bizarrely, the 10m wire managed to get so hot, at a distance of around 5 feet from the center connector, that it melted the PVC! I had to snip the wire there. Even 1500w at the max current point must be ballpark 8 amps, and #22 wire can easily handle that without melting. And if the issue was high power, then the wire should have melted towards the center where the current highest…

    Well 10m is fairly dead right now, so being short on time I resoldered the now-slightly-shorter 10m wire, covered it with electrical tape and taped it to the spreader.

    Once I get some time, I’ll see if the 30m loads up by itself reasonably on 10m. If so I”ll swap the 10m wire for a 12m or 6m one…

    What an odd problem the 10m wire melting… maybe a falling branch damaged that wire to the point where only one strand was left?
  3. W9IQ

    W9IQ Ham Member QRZ Page

    Two things to think about when diagnosing the failure.

    The current on the wire will flow through the skin of the wire, not its full cross section. As frequency rises, the skin effect becomes more pronounced. This effectively reduces the current handling capacity as the frequency rises. The fact that it overheated at a PVC insulator may indicate that the thermal resistance at that point was sufficient to cause the wire to melt due to the trapped heat.

    The antenna will have a standing wave of current. This will be present on 10 meters but it can also be parasitically induced by another element operating on another band causing a current antinode (maximum) to be at a location you may not originally anticipate.

    Good luck with the project. Thanks for documenting your progress and results.

    - Glenn W9IQ
    Last edited: Sep 19, 2021
    M0AGP likes this.
  4. W9IQ

    W9IQ Ham Member QRZ Page

    Per my prior post, the 22 gauge wire RF resistance will be about 0.7 ohms per meter of length on the 10 meter band. From there, you can calculate the heating effect of the current standing wave.

    - Glenn W9IQ
    M0AGP likes this.
  5. M0AGP

    M0AGP Ham Member QRZ Page

    A quick update on progress on the 160m inv L + 80m full size vertical, for anyone interested.

    I had expected to have impedance matching challenges, but in fact due to the lossiness of the installation (mainly earth), the SWR ends up being manageable for the band portions of interest, which are CW and data.
    For the 160m inv L, resonance (X=0) is at about 1.845 with R = 28.8 ohms; min SWR is 1.73 at about 1.828. As my amp can act like a tuner I can easily operate with up to 2.5 or even 3:1 SWR so no need for a matching network here for the low end of the band.
    For the 80m vertical, resonance (X=0) is at 3.541 with R = 42.6; min SWR is 1.14 at 3.533.
    For low SWR focused people this looks great, but if I model this with a perfect ground, R = 34 not 43 ohms, so it is ground loss that is providing a convenient SWR.
    I will probably add ground radials, and expect R to go down a bit and SWR to go up - better!

    Here was an odd thing: I tuned for resonance by adjusting the elevated radial lengths and ran into a very surprising behavior, which is now fixed.

    I initially had one of the elevated radials doing a zig-zag to try and fit the radial wire in the intended spot. As I reduced radial length, I found that R went from like 28 up to 41!!
    I assumed the zig-zag was causing something weird to happen, so rerouted the radial in a way that eliminated the zig-zag. The large R value disappeared with no change in radial wire length.
    I suppose that there had been some cancellation happening due to the zig-zag effect. So this is a case where, unlike ground mounted radials, you want to have a reasonably straight wire I think.

    One other modification I made was radial related. Initially I was having one radial along the back fence using electric fence insulators (extremely easy to install for radials!) and the other was going to rise up and be attached to a large oak at around 10 feet up. But this produced an opening angle of the radials of around 50 degrees (with 180 degrees being the desired angle).

    I found that by rerouting this radial, I could get an opening angle of around 135 degrees or so for a distance of a bit over 45 feet, and then I followed the property boundary, using conveniently placed oak trees with electric fence insulators at around 7 feet AGL. Vaguely averaging by eye if you had to choose an equivalent straight line, the opening would be somewhere around 90 degrees now, and the feedpoint R also dropped when doing this, which is a good thing!

    When testing the 160m inv L, I used FT8 and PSKreporter to show me who could decode me (and who I could decode). The result was hilarious: I could only hear Europe and one US station. But I was being copied along the entire US east coast, in numerous South America countries and most amazingly at a workable strength in Australia by VK5ARG, a club station!

    I expected I would need a receiving antenna, but this very extreme demonstration of the effect of noise drove the point home! I emailed VK5ARG, expecting that they must be using phased Beverage antennas or something, but in fact they were using an extremely fat vertical antenna....

    On my noise environment: on 160m after sunset, with my CW filter at 200Hz, I get about S3 noise. And of course wider bandpass filters result in higher levels.

    Next stop: learning about the K9AY receiving antenna...
  6. W1VT

    W1VT Ham Member QRZ Page

    A good small lot receiving antenna has a broad main lobe and a sharp null of the back. By varying the location and azimuth it may be possible to hear well in most directions.
    I have eight flag antennas strung up between trees in my back yard.
    M0AGP likes this.
  7. M0AGP

    M0AGP Ham Member QRZ Page

    Wow eight flags - do you use a remote switchbox? And what kind of dimensions are they? It seems you can scale a number of these receive antennas up or down and they still work.
  8. W1VT

    W1VT Ham Member QRZ Page

    I have a pair of remote switch boxes. One set is N S E W and the other is offset from that by 45 degreees.
    M0AGP likes this.
  9. M0AGP

    M0AGP Ham Member QRZ Page

    Zak what are the approximate dimensions of your flags? I'm thinking the 25 foot height of the K9AY + keeping the thing away from the Tx antenna and the house (not sure that is even possible) would result in something visible to nosey neighbors.

    So far my camou-painted 6BTV, fan dipole, full size 80m vertical and 160m inverted L are pretty much invisible to the naked eye. But a mast popping up to 25 foot height away from the trees would be eye-catching...
  10. W1VT

    W1VT Ham Member QRZ Page

    I have some 10x22ft flags that cover 160 to 30 meters in noisy directions. I have larger ones in quiet directions.

    My neighbors don't care at all about my antenna wires. They are hidden in plain sight by my flowering shrubs which bloom all season.

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