Dave and Grant,
Interesting discussion. I am thinking about stringing up a 160 vertical
hanging off the side of the tower and am thinking about 4 elevated radials. I
don't have the real estate to accomodate full length radials so they ends will
be angled off to the side at about 70 % of their length. Will look like a
german swastika hihi. I was wondering how to tune the radials so hope to pick
up some tidbits on how to do it from this discussion. Dave, how high is your
vertical section ? I can't achieve a full 130 feet quarter wave on 160, will
have to settle for 115 feet and use a loading coil. I am thinking for the
radials to be 10 feet high. I read that 4 elevated radials are as good as a
lot of radials on the ground.
Bob
K6UJ
> On Aug 29, 2020, at 5:08 PM, Grant Saviers <grants2@pacbell.net> wrote:
>
> Dave,
>
> I went thru this with my 160m elevated T, measuring current in each radial.
> With my MFJ RF current meter calibrated on several ranges I found the 125ft
> 10ft up radial currents varied about 5 to 1. None at zero though, which
> would be my first investigation also. The vertical is a #13 Davis so easy to
> measure amps and the radial currents summed to very close to the vertical
> current. My radials are 12.5ga Al electric fence wire. Your radial currents
> sum to 46.1 vs the 41 you expected, maybe close enough.
>
> I have some R65 still on the ground, prior use AM BCB and the bottom 2x 10ft
> are welded on full coverage steel sheets, and welded together flanges. I
> thought the sheets were there as anti-climb, but perhaps more as conductors.
> I was told welding flanges was common BCB practice, I would assume mostly on
> high current sections.
>
> At your low power errors might be significant, IIRC I was using about 50w,
> not much of a concern at 1.8MHz. Your presence might be significant.
>
> In my case radial proximity to a steel building, ending near a tower, routed
> thru numerous trees and a couple in the open is why I think the currents are
> different (highest to the tower, then nearest the building). Exactly how I
> would equalize the currents is unknown to me.
>
> There is some "expert" wisdom that fairly strongly advises making radial
> currents equal, although I haven't seen analysis to back up those assertions.
> Arguments were, big pattern issues and I^2*R losses. The extra I^2*R losses
> are easy to calculate and were insignificant. If you don't believe in my
> NEC4.2 modeling then anything can be claimed.
>
> So I took my T model and inserted a source in each radial equal to the
> measured current and checked the pattern, swr, etc. Maybe 1db skew from
> azimuth symmetry and no significant change elsewhere. My model with my 3
> other towers present shows more skew. So, end of my concern until I'm
> educated more about this issue. My path losses due to fairly dense forest is
> much more of an issue, per the recent QST article, but not there isn't much I
> want to do about that.
>
> Grant KZ1W
>
>
>
> On 8/29/2020 11:23, Dave Sublette wrote:
>> Having followed the Inverted L discussion and learning lots, I decided to
>> document my 160 elevated ground plane antenna. It has been up for thirty
>> years. I replaced the feedline last year. It is 200 feet from the shack
>> and I ran a new run of half inch superflex through an underground conduit.
>> I have been very happy with the performance of this antenna. I work the
>> CQWW160CW contest QRP, an honest five watts output and place in the top ten
>> in North America and top 20 in the world. If I weren't too old to stay
>> awake a little longer, I might do better. Hi
>> I hate long detailed emails, but I think you should have an idea of how the
>> thing is made before I ask for your advice. The tower is Rohn 45 with a
>> length of four inch irrigation pipe at the top to adjust for best SWR. SWR
>> at the moment is 1.6:1. The feed point is a Rohn Broadcast antenna section
>> with the insulators at 27feet. Eight Radials are attached to an aluminum
>> plate which is insulated from ground also. The SO239 connector has three
>> #12 wires, one to each leg. The radials are each tied off with an
>> insulator and the pigtail fastened to the aluminum plate with threaded, #10
>> screws and terminals. The radials are #13 from Davis RF, insulated.
>> So I took my KX3 and a battery pack up to the feed point and took data in
>> each wire, while transmitting with 5 watts. I expected to find the RF
>> current to be divided in thirds in the radiating element and by 8 in each
>> of the radials. I had previously calibrated my RF current meter at five
>> watts to a dummy load on the bench.
>> On my RF Current meter, 5W to a dummy load yields 41 micro amps on the
>> meter.
>> If the system works as I expected, I should see13.66 microamps in each wire
>> to the tower and 5 microamps + in each radial.
>> What I measured was unequal currents in the three wires to the tower, 13.1,
>> 9.8, and 15.6 microamps. Close on the total, but not equal.
>> The Radials measured, 9.8, 6.4, 0,1.8, 26.2, 0, 1.9 and 0 microamps.
>> My first thought is to go back up and clean up all connections, replace
>> crimp terminals on the radial ends and re-measure.
>> Additionally, I should be able to take an antenna analyser up (VA-FA5) and
>> measure the behavior of each radial. I expect to see differences because
>> of variations in droop angle, nearby objects (barn), etc. But I don't know
>> what to think about the radials that measure 0 and less than 2 ua.
>> Does anyone have any further suggestions for how I should go about testing
>> and troubleshooting this system?
>> I am totally surprised by this result, since this antenna works (but we all
>> know that this is an inaccurate and unscientific statement).
>> Thanks for your patience, if you made it this far.
>> 73,
>> Dave, K4TO
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