Thread: Rca ant751
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Old 25-May-2017, 4:12 PM   #61
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Is there a way to figure out the overall gain of the antennas you tested for their VHF High/UHF
With the data as previously presented, no. The comparable data from a reference antenna of known characteristics would need to have been included so that the mathematical comparisons could be done. See image and discussion following.

...and FB numbers
F/B ratio could be measured by performing the same set of measurements with AUT aimed 180 opposite boresite. I didn't make that particular measurement using the linear frequency sweep procedure plotted above although I did do 360 rotational patterns which would discern this behavior though for specific frequency cuts.

Here's a sample that compares the RCA7511 to our clip-on VHF module which is tuned to 195 MHz, the center of the high-VHF band. See notes following.

1) A hardware failure in my circuit that synchronizes the SA sweep to the rotor's start of rotation forced me to manually trigger the SA sweep using a manually activated micro-switch in conjunction with activating the rotor. Consequently, the data may be easily skewed by as much as a second or two (7.5-15 degrees). Kindly overlook this shortcoming, it's particularly noticeable in the 7511's pattern.
2) The data was normalized so that the maximum point in the paired data set was set to "0" and the rest of the data was adjusted by the same amount for the purposes of this comparison. Using Excel, there are, of course, a good number of possible data display methods possible, this happens to be the one I chose as I attempt to enhance my feeble Excel and VBA skills.
3) The VHF module, if balun and insertion loss adjustments were included (they aren't at this point) would be about as close to a reference dipole as a no-budget measurement effort could produce. I may explore this later and then compare it to the BicoLOG just to see how close they are to each other. In the meantime, I'd assume a 1 dB insertion loss @195 MHz and that will get you better than in the ballpark (probably into the infield) for an estimate.
4) Using the above described F/B definition, the F/B of the dipole is measured a almost zero, the 7511 is measured at about 15 dB.
5) Using the above described gain definition and including the estimated insertion losses, I'd estimate that the forward gain of this antenna, as tested, is ~ 5 dBd (7.15 dBi) at 195 MHz.
Attached Images
File Type: jpg VHF1_vs_RCA7511_195MHz.jpg (115.3 KB, 2153 views)
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Last edited by ADTech; 25-May-2017 at 4:15 PM.
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