Tom,
Yes.
Just sweeping from the end of the coax in the shack should show the same dip of
R at the correct frequency.
(The absolute R value (in Ohms), and X value will not be correct because of the
feedline transformation, but the frequency of the R dip is ONLY a function of
the reflector tuning.) Try it and see.
In the “early” days of tuning my 2 el 80m Yagis, I did the usual:
1.) Open the driver, then put the analyzer on the reflector element
2.) sweep the reflector (at full height) and measure resonance
3.) Compare to the model, where you have swept the reflector in the model with
the driver open (inset 1 E6 load).
4.) Retune the reflector and test again.
This is entirely un-nesseasry and this is still being used by many as a method
for tuning. Far too complicated, and also subject to the effect of the driver
tuning.
When you sweep the driven element of any Yagi with the reflector in the
circuit, it will always show a pronounced dip in the REAL R at the resonance
frequency of the parasitic element because of mutual coupling. This is
completely independent of the driver tuning. Think of this as though at the
parasitic resonant frequency, you now have 2 elements in parallel. This is
like two resistors in parallel. The resistance will drop. It has to because
the drive energy is now being shared by 2 elements!
As for the accuracy of EZNEC for tapered elements, this is the benefit of this
tuning method! It is independent of the specific element characteristics
because you are just tuning for the RELATIVE relationship between the reflector
tuning and the preferred centre SWR QRG.
Look at your model. Notice the relationship between the frequency of min R
dip, and max Gain and Max F/B frequency. In a reflector model, obviously these
are above the min R dip.
The key is that YOU decide what frequency to tune the driver at. This is
independent of the reflector tuning. You can tune the driver (min SWR)
wherever you want. This is just a matter of matching.
1.) If you tune the driver at the Max gain QRG, the feedpoint impedance will be
very low, and the 2:1 SWR bandwidth will be very narrow.
2.) If you tune the driver at the max F/B QRG the impedance will be higher and
the SWR B/W will be more broad.
3.) if you tune the driver well above the min R dip, the SWR bandwidth will be
very broad, but you will be sacrificing gain.
This is the design parameter that you play with to balance max gain, F/B and
SWR bandwidth for a given design with the modeled element spacing and loading.
What is important is that using the model, you will see what frequency the
driver needs to be tuned for min SWR relative to the reflector tuning.
The tuning of the driver is accomplished with a L network, hairpin or gamma
match or whatever will transform the complex impedance to match your feedline
at your chosen QRG. Tuning the driver is a separate process and is independent
of the reflector tuning.
So, on the tower to do a FINAL tuning of the Yagi:
1. Sweep the driver and find the min R dip
2. Adjust the reflector until it matches your preferred model min R dip
frequency.
3. Based on the model, tune the driver for the desired QRG relative to the min
R dip. If the analyzer has been calibrated to the feed point, or is AT the
feedpoint, then you can build an L network or hairpin to match based on the
measured R+jX value.
Using this method I have with confidence tuned my 2 el 80m and 2el 40m Yagis,
and also my 2 el 160m parasitic array that uses the shunt fed tower as a driver.
It is entirely unnecessary to do any field testing to “check” for max F/B or
max gain, since this method assures correct tuning. You know with confidence
that the Yagi is tuned exactly X kHz above the reflector tuning, which is what
your model has shown to yield the parameters you want.
As i said before, the common mistake that guys make is they measure IMPEDANCE
instead of the real R with the analyser.
I explain how i did this in these papers I wrote for my 2 el 80-40m Yagi, and
the 2 el 160m Parasitic array:
80-40m Yagi:
<https://drive.google.com/file/d/1IksvND0LfeSBcH6j7UEWkyjkIvexDIgX/view
<https://drive.google.com/file/d/1IksvND0LfeSBcH6j7UEWkyjkIvexDIgX/view>>
160m 2el parasitic array:
<https://drive.google.com/file/d/1eVklkZtlJW93JIJ9oDqbCyl-Jkik6YEX/view
<https://drive.google.com/file/d/1eVklkZtlJW93JIJ9oDqbCyl-Jkik6YEX/view>>
73, de steve ve6wz
> Steve:
> That is indeed useful information. I have done a fair amount of modeling
> and never noticed that effect, but I just did a sweep in the model of my
> 2 el. yagi and you are absolutely correct. The only thing I am not sure
> about is, if I put an analyzer (I use the FAVA-4) on the end of a random
> length
> of feedline coming down my tower from the antenna, and it shows a dip in the R
> at say 13.950, indicating the frequency of resonance of the reflector, would
> I see
> a pronounced dip in the R at the same frequency if I measured it again after
> changing
> the feedline to some other random length?
>
> In your experience, how accurately do you feel EZNEC can predict the resonant
> frequency of a tapered element?
> I guess if the answer is very accurately, then I probably should not waste my
> time measuring it, just build
> the antenna to fit the model and go with it
>
> Thanks a million for your reply and input. It's definitely a keeper.
>
> 73
> Tom
> K0SN
>
>
> On Thu, Apr 23, 2020 at 12:01 PM VE6WZ_Steve <ve6wz@shaw.ca
> <mailto:ve6wz@shaw.ca>> wrote:
> Tom,
> Yes. there is a MUCH simpler way to do this without climbing the tower.
>
> I have tuned a number of hombrew Yagis and vertical parasitic arrays, and
> used to think I needed to isolate the driver, then measure its resonance.
> That is not required.
>
> Because of mutual coupling, the REAL R (NOT IMPEDANCE) of the driver
> (feedpoint) will show a pronounced dip at the resonant frequency of the
> parasitic element. This dip in real R is independent of the tuning of the
> driver. You even can sweep the Yagi from the ground and it should still show
> a pronounced dip. (it would be preferred to sweep it at the driver, or
> feedline corrected to the driver). Make sure to sweep well above and below
> the central QRG. (like 13 Mhz to 15 Mhz.)
>
> Look at your antenna model to confirm this. In your antenna model, if you do
> a frequency sweep and look at the output, you will see the dip in the real R
> at the frequency that the reflector is tuned to. On the tower, you want to
> see this same dip in R at the same frequency. This effect is a bit more
> complicated on a multi-element Yagi, but on a 3 el Yagi, you will still see
> the dip in the R at both the Reflector tuned QRG and the director QRG.
>
> The mistake some make with this analysis is they look at the IMPEDANCE on the
> analyser. That is wrong. We see the dip in the real resistance. ie. the R
> in Z= R+jX. Most analyzers will plot R vs frequency.
>
> 73, de steve ve6wz
>
>
> > On Apr 21, 2020, at 7:02 PM, Tom Hellem <tom.hellem@gmail.com
> > <mailto:tom.hellem@gmail.com>> wrote:
> >
> > I would like to measure the self-resonant frequency of the reflector on a
> > home made 20 meter yagi. It seems obvious the best way would probably be to
> > open it up at the center and make it into a split dipole element and hook
> > up my analyzer. However I would like to avoid cutting it and dealing with
> > splicing it back together again. Can anyone suggest another way to
> > accomplish my objective? The reason for doing this is because it is
> > apparent from modeling in EZNEC that the
> > length of the reflector is critical to the performance parameters of the
> > antenna, e.g. gain, F/B, bandwith, etc., and it looks to me like it doesn't
> > take much of a change in the length of the element
> >
> > Thanks in advance for any replies.
> >
> > Tom K0SN
> > Missoula, MT
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com <mailto:TowerTalk@contesting.com>
> > http://lists.contesting.com/mailman/listinfo/towertalk
> > <http://lists.contesting.com/mailman/listinfo/towertalk>
>
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
|