Topband: Temporary antenna suggestion for 160
grants2 at pacbell.net
Sat Dec 21 16:37:29 EST 2019
I think the problem with elevated radials in 4 squares is the mutual
coupling and the necessity that the radial current and impedance be
equal. Otherwise the pattern is distorted.
Having measured my 8 125' elevated radials there is a significant (>2:1)
current imbalance in them due to several factors - towers, trees,
buildings, stuff in the ground, etc. I modeled this by having 8 sources
in the radials of the measured currents. My modeled pattern skew is
about 1db, so I am a bit skeptical of the CW for exactly balancing
radial currents for a monopole vertical. (see also N6LF modeling of
half circle radials).
After calibrating (essential!) my MFJ RF clamp on ammeter, the measured
antenna current and sum of radial currents were equal within 2%, so the
choke worked. The MFJ can only measure amplitude. Someday I will
investigate how the current phase is different in each radial referenced
to the vertical. Has anybody done that?
On 12/21/2019 17:10, Mike Waters wrote:
> Hi Frank!
> Thank you for sharing this experience. This is interesting, because (as you
> are probably aware of) no less a guru than N6LF published results showing
> that with a single antenna, four ??/4 elevated radials were nearly identical
> in performance to 120 on the ground. This is true *only* if RF was
> prevented from either flowing into the lossy earth *or* back along the
> feedline shield (thus detuning the elevated radials, since the shield would
> try and act as a radial and couple to the lossy earth).
> Was anything connected to ground at the feedpoints? And what type of coax
> feedline choke unun did you use at the feedpoints? Perhaps there are
> factors in an array vs. a single vertical that would explain your results,
> but I can't think of any.
> 73, Mike
> On Mon, Dec 16, 2019, 3:04 PM <donovanf at starpower.net> wrote:
>> Hi Mike,
>> Years ago my 4-square transmitting array used "gull-wing" elevated
>> radials sloping 45 degrees from the feedpoint at ground level to about
>> ten feet high.
>> When I replaced the radials with sixty 120-foot radials laid on the ground
>> I had to shorten the verticals by about five feet to maintain resonance,
>> suggesting that the current at the bottom five feet -- or so -- of the
>> was attenuated by the sloping radials in close proximity to the verticals.
>> As an aside, the performance of the array improved dramatically...
>> *From: *"Mike Waters" <mikewate at gmail.com>
>> *To: *"thoyer" <thoyer1 at verizon.net>
>> *Cc: *"topband" <topband at contesting.com>
>> *Sent: *Monday, December 16, 2019 8:52:41 PM
>> *Subject: *Re: Topband: Temporary antenna suggestion for 160
>> It was just pointed out to me that I neglected to mention that the
>> feedpoint on my 160m inverted-L was much lower than 10 feet high!
>> The tuner sits on the earth, and the two wires go straight up from that to
>> the insulator block holding the antenna and the radials, which is less than
>> 4 feet high.
>> From that point, the two radials angle upwards at roughly 45?? (?) to nearby
>> trees, and level out at 10' high to the North and to the South all the way
>> to the ends. (The South radial zigzags back and forth since the distance
>> from the base to the neighbor's fence in that direction is less than 1/4
>> I had photos of it online, but w0btu.com crashed. Looking for a place to
>> upload it to.
>> I hope this makes sense. Sorry for the lack of details below.
>> 73, Mike
>> On Sun, Dec 15, 2019, 8:22 PM Mike Waters <mikewate at gmail.com> wrote:
>>> Do the inverted-L, but use at least two 10' high 1/4 wave radials.
>>> Do NOT use an RF ground rod, or any radials on or near the earth. Just
>>> connect the coax shield to the junction of the radials and any remote
>>> tuner. At that point a good choke balun is necessary.
>>> Leaving out the choke or grounding the shield will result in very poor
>>> 73, Mike
>>> On Sun, Dec 15, 2019, 7:04 PM thoyer via Topband <topband at contesting.com
>>>> With only 9 more to go for DXCC on 160 and all of the recent posts about
>>>> how good the band has been recently "best in years....) I find myself
>>>> antenna for the low bands and cringing after each post on how good the
>>>> band has been.
>>>> Options - I have a 45' tower with TH6DXX, 6m and 2m yagis. I could
>>>> string a makeshift inverted L with about 45' vertical and around 100'
>>>> horizontal. This I could string up in a few hours. the Horizontal
>>>> would be pointed south. Not the best of configurations but that's what I
>>>> have to work with. ...
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