As a matter of information I will pass along an equation for calculation of
'equivalent diameter' for triangular towers.
Somewhere I got the following formula for a cylindrical equivalent for
triangular tower like Rohn 45.
Equivalent Cylindrical Diameter = 2 * CUBEROOT [(T * F^2)/2]
where T = tube diameter and F = Face width.
For Rohn 45, T = 1.25" and F = 18", so cylindrical diameter equivalent
I would expect that Rohn 25 would have a smaller Equivalent Cylindrical
Diameter and your estimate of 12" diameter may be high.
I have used this formula to calculate Equivalent Cylindrical Diameters for
Rohn/Spaulding HDX which I have used in modeling [HyTower] and it seems to
provide results that are close to what is measured using a General Radio
1606A RF Bridge at the base.
My experience with an MFJ259B to make 160m measurements has not been
encouraging. It may be that I am too close to BC stations that contribute to
the signal that is being read by the instrument.
I don't use it for 160 measurements any more, but rather lug out the 1606A
and signal generator [the MFJ259B is OK but somewhat frequency 'agile'] and
a detector such as an IC-706. For a 120 foot tall tower with an insulated
base a value of 10 -j29 ohms for 1.7 MHz at the base is probably within the
accuracy expectation of the MFJ instrument, but the R component seems low.
Measuring the frequency [or frequency range] at which the X drops to zero
should give you an estimate of the resonant frequency of the tower. <it may
be that you have to settle for determining what the frequency range is over
which the X is low, but not zero, and where the X value begins to increase.
This change from lowest value to higher value provides a clue as to where
the sign of the reactance is changing.>
I don't know how to assess the 'value' of the readings at the other
frequencies. A 120 foot tall tower certainly could have a 30 +j100 ohm input
at the top end of 75 meters since the 120' length is in the range of 1/2
wavelength on 75 meters. [depending on the effective cylindrical diameter of
the tower]. Frankly, I would have expected the input impedance on 75 meters
to be closer to 250 +j300 ohms if you are really close to a 1/2 wave.
The excitation methods suggested by Frank and others probably are the
quickest way to get a fix on things if you don't have a real RF bridge.
> -----Original Message-----
> From: AB5MM [mailto:email@example.com]
> Sent: Monday, September 17, 2007 11:40 AM
> To: firstname.lastname@example.org
> Subject: [TowerTalk] 160m Vertical
> Thank all of you for the answers and suggestions so far in
> this hair pulling project called "Matching at 160m Vertical".
> Larry N8KU, John KK9A, Carl KM1H, Bob W5AH, Rodger KI4NFQ,
> Frank W3LPL, Arlan N4OO, Jeff N8CC
> I need to answer a few questions:
> (1) The SWR analyzer(s) MFJ-249 and MFJ-259B.
> (2) We are also using a Eico 170 GDO.
> (3) The ground radials are located on top of the soil. I made
> a aluminum grounding buss bar as a central location for all
> ground radials to terminate at a common location. The buss
> bar is bolted directly to the tower base at around 6" above
> ground level.
> (4) Sweeping the tower directly at the feed point (short RG-8
> pig tail) I do not have a dip anywhere in the 1.8 to 2.0 MHz range.
> It does show a slight dips at;
> 1.739 MHz with a swr of 3.7/1 with X=29 & R=10.
> 4.254 MHz, 4.6/1 swr X=100 R=30
> 4.734 MHz, 4.0/1 swr X=28 R=86
> 5.670 MHz, 3.3/1 swr X=29 R=80
> 8.665 MHz, 2.2/1 swr X=49 R=36
> The formula for a 1/4 wave length radiator is 234/fo. I
> really think that is for a reasonably sized wire antenna. The
> Rohn 25g is ~12" in diameter, which would make the Q a little
> lower (wider band width) as well as shift the resonance down
> a few kc. I think we are very close to the correct height, if
> not a little short at 120 ft. - 6 inches. Do any of you think
> the guy wires are adding a capacitive loading component? Is
> that what's throwing a wrench in the works?
> I'll sweep the tower this afternoon with the GDO to see if I
> can get any indication of resonance in or around the 160m
> band. The path from the shack to the tower is well worn now :-))
> Steve AB5MM
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