Interesting. w8ji also mentions that a bias misadjustment may cause excessive
harmonics and mentions the problem Sam refers to (frequency selective
loads..i.e. high Q). He says:
"Quiescent current (bias) in the RF amplifier section is adjustable. This
adjustment directly affects output signal harmonic content. Harmonics are worse
with low supply voltages, and with low impedance loads./Be sure you check the
harmonics as outlined below, with a 1/4 wl open-circuit stub!!/
Excessive harmonics can cause severe errors in measurement of
frequency-selective loads, even when dummy-load SWR tests appear perfect. Loads
most sensitive to harmonic-induced errors include, but are not limited to,
antenna tuners, tank circuits, very short resonant antennas, and distance to
fault and stub length measurements. If you notice something "funny" going on
with a stub measurement, it may be a fault of incorrect bias."
Tom wrote up a method for checking harmonics/bias at:
http://www.w8ji.com/mfj-259b_calibration.htm
-kevin
On 12/7/2010 8:27 AM, Sam Andrews wrote:
> One possibility to explain the readings on the MFJ instrument when it is
> looking into the antenna vs. when it is looking into the tuner input . . .
>
> Usually the Q of a tuner that is tuned to a specific frequency is higher,
> and the input bandwidth is narrower, when compared to the input Q and
> bandwidth of an antenna (or dummy load). If the signal source of an
> instrument such as the MFJ is not as clean (i.e., spurious nearby
> off-frequency signal levels are higher) than a typical transmitter, one
> possible effect is that these spurious signals would be reflected when
> applied to the hi-Q tuner input (and detected by the MFJ as reflected
> signals), while being absorbed by the antenna (not reflected).
>
> This opinion is not based on actual experience with the MFJ, but on my
> experience working on UHF transmitters with spurious signal problems. When
> connected directly to a dummy load, those problems didn't show up, but did
> show up when connected to the dummy load through a frequency-selective
> circuit, such as a resonant cavity or duplexer). Same principle, I believe.
>
>
> 73, Sam AE5L
>
>
>
> ----- Original Message -----
> From: "jim feldman"<mtnredhed@gmail.com>
> To:<towertalk@contesting.com>
> Sent: Tuesday, December 07, 2010 1:45 AM
> Subject: Re: [TowerTalk] who's "right" (a tuner question)
>
>
>> ok, a little more testing. I set up on 7.230. I tuned via the noise peak
>> and then used low power to dial in. Both the TS-940 and the meter on the
>> antenna tuner agreed the match was at or near 1:1. Next, I
>> then disconnected the feed from the transceiver and substituted the
>> mfj-259.
>> I dialed the vfo on it to 7.23 (or as close as I could get it to stay) and
>> the swr needle on the analyzer was by the infinity mark. I then plugged
>> the
>> antenna feed line directly into the mfj and got a 2:1 b/w of 7.23 to 7.33
>> that dipped to 1.7:1 in the middle.
>>
>> observed: MFJ-259 is ok connected directly to antenna, not ok when feeding
>> the T match. I would have to say that local RF sources are not the cause
>> of
>> the difference. I also verified that the swr as measured by the
>> ts-940/tuner was the same at full power as it was at 7 or so watts. I'm
>> not sure why the tuner and the analyzer don't get along. I am settled on
>> the fact that the inline meters are likely correct.
>>
>> The MFJ-259 may be ok for some uses, but driving this T match and antenna
>> isn't one of them.
>>
>> jim
>> w6jmf
>>
>> On Mon, Dec 6, 2010 at 5:09 PM, jim feldman<mtnredhed@gmail.com> wrote:
>>
>>> It took me a little while to understand how a strong BCB station might do
>>> this, but I assume it's because the MFJ "sees" the signal as reflected
>>> power? I didn't (and don't) see that if I plug right into the antenna
>>> feed
>>> line. Someone suggested dialing it in via the tuner/transmitter swr
>>> meters
>>> and then checking the match via the MFJ289. I'll try that this evening
>>> and
>>> report back the results (little embarrassed I didn't think to try that).
>>>
>>> I'm not saying that isn't the case but I've been successfully using the
>>> analyzer for some months on vhf antenna projects with quite low matches,
>>> and
>>> I also periodically test the trapped vertical, and get back pretty good
>>> matches on 10 15 and 20. Even 40 is pretty good. 75 is just very
>>> narrow,
>>> but under 2:1
>>>
>>> Thanks for everyone's help
>>> jim w6jmf
>>>
>>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> 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
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|