[TenTec] Omni VI SWR Meter
Tue, 3 Jun 1997 21:08:52 -0400 (EDT)
This kind of thing is quite common. To begin with, none of your meters are
seeing 50 ohms hence the obvious, reflected power indication on the
various meters. Due to the mismatch, your feedline (coax) has current and
voltage lobes and nodes along the length. Depending on where you place the
meter will determine what it reads. In other words, you could add more or
cut off some coax and change what your meters show. To prove this, just
take about 5ft of coax and add it to your feeder and watch your meters all
change readings. This is a big bugaboo with swr meters. The only way to
accurately measure your swr is to either put the meter at the antenna or
use an exact, electrical multiple of a half-wavelength of feeder for the
frequency being checked. Both are a pain!
Try this. Hook a 50 ohm dummy load to each meter and see if they all read
1:1. If they do then they are probably all ok. Hooking all those meters in
series will place them at different points on the feeder and if the feeder
has standing waves, they will and SHOULD read differently. None of them
are necessarily correct.
Furthermore, reviews in QST prove that swr meters are usually not very
accurate. 10% error is quite common. I wouldn't be concerned about a few
tens of a percent variation.
This leads to another point. Using a "wattmeter" in a line with standing
waves present will also show considerable error. Be sure to use a dummy
load or to have a 1:1 swr indication when measuring power.
73 Steve N4LQ
On Tue, 3 Jun 1997, Dale Martin wrote:
> I am seeing a high SWR on my Omni VI meter. I have three other
> wattmeters in line (Daiwa CN-720B, Daiwa CN-101L, and an MFJ
> Wattmeter in a tuner--admittedly, not the best, but it does show
> SWR right along with the Daiwa's).
> At full power out, the Omni SWR meter reads 2:1 into a dummy load.
> The other wattmeters are showing 80 watts forward, 0 watts reflected.
> When I go to my antenna on 20m CW, I see 1.5:1; on 40m,
> with Daiwa showing 1.4:1 I see 1.9:1 on the Omni; on 80m, with
> Daiwa showing 1.6:1, I see 3:1 on the Omni.
> Any ideas as to what is going on? I would send it to
> Ten-Tec, but I don't want to jeopardize my place in line
> for the Option 3 mod.
> Besides, if it is still here then, I plan to use it on FD later
> this month.
> Thanks in advance.
> Dale Martin, KG5U
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Steve Ellington N4LQ@IGLOU.COM Louisville, Ky
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