[TowerTalk] Why did adding a radial make the input Z of my
inverted L go up?
jimsmith at shaw.ca
Tue Mar 16 15:28:33 EST 2004
I've found the reason for my puzzling results.
Just so you don't have to read the original post again, I had an
inverted L on 160 with no radials (but with coax shield connected to
ground so I actually had one radial, the coax). I started adding
radials and found the Z at the base of the insulated-from-ground tower
went down when the 1st radial was installed and went up when the 2nd one
was installed. (Coax centre conductor not connected to anything.) Z
was now higher than with no radials at all. This is better?
I had another go at the measurements. I decided to find where the
antenna was resonant as it had been purposely made too long. I
disconnected the coax centre conductor from the tower and the shield and
the 2 radials from the ground plate (we use plates here, not rods) and
adjusted the Autek RF1 analyzer frequency to find the freq where L=0 on
the presumption that if there's no L we're at resonance.
There was no frequency where L = 0. L hit a minimum of 15.9 uH at 1775
kHz. Both the SWR and Z displayed by the RF1 were minimum at the same
frequency, 6:1 and 179 ohms respectively. For the rest of this, fo is
the frequency at which L was minimum
1 Ground plate alone as described above fo= 1775, SWR= 6.0, Z= 179, L= 15.9
2 Coax braid connected to ground plate fo= 1754, SWR= 5.3, Z= 130, L= 11.8
3 As in 1 plus one 12 ft radial fo= 1750, SWR= 4.5, Z= 134, L= 12.2
4 As in 1 plus one 42 ft radial fo= 1526, SWR= 4.7, Z= 112, L= 11.5
5 As in 1 plus both radials fo= 1514, SWR= 5.5, Z= 112, L= 11.7
So, I have the answer to my question. The 42 ft radial drastically
altered the "resonant" frequency of the antenna. Consequently, it is
not surprising that impedance measurements all made at the same
frequency would go up when the longer radial was connected.
What is surprising, at least to me, is that buried radials can affect
the resonant length of the antenna so drastically. (They're #14,
stranded, insulated, 2-3" below the surface.)
For the sake of completeness:
6 As in 2 plus both radials fo= 1510, SWR= 3.9, Z= 107, L= 11.2
7 As in 6 plus 10 more radials fo= 1521, SWR= 4.4, Z= 82, L= 8.7
The 10 more radials range in length from 8 ft to 28 ft and their ends
are 2 ft apart. Why 2 ft instead of recommended 8 ft for 160? I've
found the L works better on 40 than my dipole so based the spacing on 40m.
I have 7 more radials to install. At that point, my radial space will
be full. The new ones will all be about 28 ft.
I'm gratified to see that Z is down to 82. I'm hoping that the
additional radials might bring it down to 60 or so.
Now I have 3 new puzzlements.
1. Why can't I find a frequency where L = 0?
2. Why did the the SWR go up when Z went down when comparing case 7 to
3. When I get the rest of the radials in and cut the flat top to length
how will I know when I've got the right length?
Just for fun I connected an unterminated wideband (10 MHz) true rms
voltmeter (HP 3400A) to the shack end of the coax connected to the L.
Got a rather surprising and very steady 12V. Could be 60 Hz pickup, I
suppose. I tried again with a 50 ohm termination and now get a more
reasonable 85 mV which is bouncing around 2-3 mV, presumably due to BC
station modulation. This may be the source of the SWR puzzlement and
maybe even the "L doesn't go down to zero" puzzlement.
I suppose I could hump the TEK 535 scope out to the tower and measure V
& I magnitude and phase of power fed from Tx to get around the BCI, if
that's what my problem is.
I did make 99Qs/19 mults in the recent CQ 160 SSB LP with no radials,
just the ground plate. I'm looking forward to the beneficial effect of
Any and all comments appreciated.
73 de Jim Smith VE7FO
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