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Re: [TowerTalk] 160m Vertical

 To: "'AB5MM'" , Re: [TowerTalk] 160m Vertical "Tod -MN" Mon, 17 Sep 2007 15:20:01 -0500
 ```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 =11.7446" 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. 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. Tod, K0TO > -----Original Message----- > From: AB5MM [mailto:ab5mm@9plus.net] > Sent: Monday, September 17, 2007 11:40 AM > To: towertalk@contesting.com > 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 > > > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > > _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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