Upon tuning the antenna for a specific frequency, we used the measurements specified in the Ramsey antenna manual listed above.
The manual states that the upper section of the vertical element is to be measured at 16 1/8" or 409.8MM. While the transmitter was transmitting at low power, the
high SWR protection circuit was preventing the transmitter from transmitting.
Upon extending the 5/8 wave to full length, the transmitter began transmitting just enough to start getting some readings. The transmitter has an adjustable SWR warning threshold from 2.0 to 3.5. So obviously, we set it to 3.5 to get the transmitter to stay transmitting. It also has its own built in SWR meter and we also have a Diamond antenna SX-200 1.8MHz to 200MHz SWR/Power meter in line.
Now as we had the antenna fully assembled near the ground and making adjustments to the upper section, the SWR started to drop down from 3.5 with each 1/8" adjustment we made. We eventually got it down to 1:9 and with the next 1/8" reduction we made in length, the transmitter went into protection mode again and would not transmit again until we increased the length again.
We ended up with a final SWR of 2:0, Also, if we remove the SX-200 from the transmission line, the transmitter goes into high SWR alarm mode again.
The antenna is mounted on a mast 20 feet above the roof, the antenna system is grounded via a 5 foot ground rod deliberately driven into the ground in 2 foot deep hole. So basically the top of the ground rod is 2 feet below ground level and the hole was filled in with rocks and dirt with a dip to allow rain run off to get trapped and sink into the hole for good ground conductivity.
Since we can not get the automatic SWR alarm to stop shutting off the RF power amp, how can we get this SWR down to 1:1 or at least below 1:7?
Antennas ideally need to be tuned in their final operating position. Since it's roof mounted, I suggest lowering the mast, adjusting, hold it up, test, repeat until done. Then secure it.
Post by ogrevorbis on Jan 28, 2014 18:30:57 GMT -6
Make sure you're tuning the antenna in an open area with at least 1 wavelength away from any objects (including you). At 107.1 that would be about 10 feet. The farther away from objects, the better. 5/8 wave at 107.1 is 69 inches. Your vertical should be aprox that long.
Make sure you're tuning the antenna in an open area with at least 1 wavelength away from any objects (including you). At 107.1 that would be about 10 feet. The farther away from objects, the better. 5/8 wave at 107.1 is 69 inches. Your vertical should be aprox that long.
Inches 68.90756302521008 Got ya. When the weather gets warmer we're going to take the antenna down and re-measure the over all length, right now there's no way to tell from the ground.
We did mess with the frequency a bit to see if the SWR dropped if we went up to 107.9 or if it dropped going down to 106.1, turns out the reading at 106.1 was 1:4:1 compared to 2:0:1 at 107.1.
We know we were a little long, but as I said, since the antenna was two feet above the ground, the minute the top element was reduced another 1/8" the SWR alarm went off and the transmitter refused to transmit until the 1/8" was added again.
Post by ogrevorbis on Jan 29, 2014 18:24:45 GMT -6
I found that wavelength calculation to be good as a starting point, but most likely due to the surrounding area, you're going to need to change it. With my 1/4 ground plane antenna, I ended up having it about two inches shorter than what the calculation said. So you should probably start at about 75 inches and start shortening it.
Post by HighMountainRadio on Jan 13, 2018 19:34:52 GMT -6
Greetings Mr Ed !
I read your posted info.. Are you using a balun at the antenna's feedpoint ? If not, this could be your whole problem.. Also, RG-8/U isn't the best coaxial cable suited for the application, especially inasmuch so, that you are running a pretty long (100 Foot) run. A far better, far less lossy choice, would either be RG-9913 or LMR400 from a good, reputable source ! It's quite a bit pricier, but it is WELL worth it !! Be ABSOLUTELY certain to incorporate a homemade 'Balun' !! That will either eliminate completely, your reflected power, or at least greatly minimize it ! I have NO reflected power here.. If you opt not to use a blaun, your coaxial cable is sure to radiate and cause loss of transmission power to the antenna and present a dangerous level of reflacted power back to the transmitter, which is almost guaranteed to be what you are experiencing !
73... Spooky...
Here is a pix of my homebrew balun for FM Broadcast service:
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