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Old 03-24-2014, 02:21 PM
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DaveInDenver DaveInDenver is offline
Hard Core 4+
Join Date: Jun 2006
Location: Larimer County
Posts: 6,655

Took a few sweeps of the STI-CO antennas I have. These are not lab quality, done in the driveway with a hasty calibration and a couple of adapters. Each grid line represents a change in VSWR ratio of 0.5 and the reference lines are 1:1 (e.g. zero reflection measured at the S11 port) and 3:1.

The one cut correctly for 2m shown across the 70cm band. Looks awesome at 430MHz and pretty good generally. Sweep goes from 420 to 480MHz, center is 450MHz and each step is 25MHz. The prominent dip is ~435MHz at almost 1:1 SWR. Through the bulk of the phone part of 70cm the SWR flirts with 2:1.
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Here's a correctly cut (per formula) 1/4 wave across the same sweep. It doesn't look all that great to be honest, most of the band sits around 2:1 SWR. Main dip is ~443MHz at 1.05:1 SWR (I calculated 445MHz as my intended center). I think the noise is not due to the antenna but the feedline and connectors. Also seems there is some sort of a strong local interference that I can't find, which is partially why my plots have a sine wave imposed on top of them.
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The same antenna in my full configuration, patch cable to duplexer broken out to two antennas cut for each band. Difference in this one to the previous is the cable and MFJ duplexer, which clearly have influence. I suspect the length of patch cable at 9" probably is the main reason, but both seem to show a lot of effect of the connectors, coax, etc. Very ugly plots. Also shows my antenna is still not trimmed short enough on 70cm, but before I go cutting more off I want to figure out the duplexer dependency.
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Same as last one just swept across 2m. No surprise, very little to see, narrow frequency range, wide bandwidth and VHF. Range is 140 to 152MHz, center is 146MHz and the SWR is 1.05:1 at 146.50MHz.
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A 50 ohm resistor at the VNA output for reference. This is a crappy test load, a PL259 with a RF resistor soldered inside of it. This shows how bad the PL259/SO239 interface is at UHF. The resistor is useful out to 1GHz and is 51 ohms, e.g. should show almost a perfect 1:1 SWR (within reason) but at 70cm the connector impedance is anything but uniform. Can't blame much of this on my skills since it does show correctly at lower frequencies (you can see the SWR rising coming in from the left and continuing across the sweep).
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