Dave…
Indeed few will be motivated to implement a complex in-band switching system,
but if you are a home brew antenna builder, its not that difficult, but it is
NOT plug-and play.
I like to build antennas, so I will take the extra .5 to 2 dB that I get by
being always tuned for max performance and using HI-q coils..
Commercial Yagis, that are plug-and-play will always be compromise systems to
cover the full band. (Except something like the the SteppIR system that will
always be optimized.)
The consumer wants to put the Yagi on the tower, and have a nice low SWR from
CW to SSB with limited complexity. Building a commercial Yagi that includes
complex relay inductor switched boxes at each element to maximize the gain
across the band (multiple segments, not just CW SSB) is probably not doable for
multiple reasons.
However, there is a gain price….especially with shortened, loaded Yagis. Short
loaded 2 el Yagis really need to be tuned for optimal gain because they show
VERY narrow gain/FB bandwidth. (3-el Yagis are more forgiving)
Regarding relay reliability, on my 80m Yagi I use vacuum relays, but on the 40m
Yagi use simple high current open frame (non-RF rated) relays and have had no
failures in 18 years. High current sealed units would work too I think.
For anyone interested in why SWR bandwidth and Yagi performance are NOT
related, I really encourage you to read my paper and focus on the *modelling
section* where I show the relationship between driver tuning, SWR bandwidth,
and FB-Gain performance. Its really easy to get a shortened Yagi to have a big
SWR bandwidth, but gain performance will be crap across part of that full range.
https://drive.google.com/file/d/1IksvND0LfeSBcH6j7UEWkyjkIvexDIgX/view
<https://drive.google.com/file/d/1IksvND0LfeSBcH6j7UEWkyjkIvexDIgX/view>
Steve, ve6wz
> On May 22, 2021, at 5:19 PM, Artek Manuals <Manuals@ArtekManuals.com> wrote:
>
> Steve
>
> Thanks for the proper proper analysis...8^)
>
> Ignoring possible SWR losses as many will unlikely go to your due diligence
> on matching which I applaud..I would argue that .5 to 1.3db is hardly worth
> the physical time and effort
>
> I would also argue however on your side about the relays... If one chooses
> high quality relays, with plenty of margin on current and voltage protection
> and then properly mounts them in a well thought out (relays at the top
> screened drain holes at the bottom ) weather resistant enclosures, they will
> likely out live most operators ...8^)
>
> I use an approach similar to yours to move the resonance around on my 160 and
> 80M antennas. I have had only had one problem here in the� jungles of west
> central Florida which was my own fault ( Part of the curse of having ADD)�
> and I left the door to the enclosure open� over night� and all were exposed
> to the rain overnight . Things got a little spectacular when I dumped 1.5KW
> into the 160 box the next morning ...8^(
>
> Dave
> NR1DX
>
> On 5/22/2021 5:36 PM, VE6WZ_Steve wrote:
>> Dave et al,
>>
>> This has drifted off topic, but I dug out my 4NEC2 Cushcraft XM-240 models
>> and coil calculations.
>> (please be aware I have not checked the details of this modelling since I
>> did it some time ago, so there could be errors)
>>
>> Using Brian's coil 3.33, which is not the latest version, the original CC
>> coil with 68 turns of 12 ga wire on a fibreglass form gives a Q of 258 on 7
>> MHz and a resistance of 1.5 Ohms.
>> A high Q coil made from 3/16� Aluminium tubing, 11.5 turns, 5� diameter and
>> 5� long gives a Q of 890, and a resistance of .6 Ohms.
>> (these two input values do not yield identical inductance, but are close
>> enough for estimation)
>>
>> In 4Nec2 I model the Yagi at 21m high.
>>
>> For the original CC coils I insert 1.5 Ohms in series at each of the 4 coils
>> to account for the loss.
>> This yields the following gain:
>> 7.010-10.8 dBi �this is peak gain (SWR 2:1)
>> 7.100-10.2 � � this is the 1:1 SWR point at resonance (R=48.6 Ohms)
>> 7.200-9.48
>>
>> Using the HI-Q coil I insert .6 Ohms in series at all 4 coils to account for
>> the loss.
>> This yields the following gain:
>> 7.010-11.3 dBi this is the peak gain (SWR 2:1)
>> 7.100-10.52 � �This is the 1:1 SWR point.
>> 7.200-9.63
>>
>> So I calculate the gain advantage of the HI-Q coils to be about .5 dB.
>> The �penalty� for getting that big bandwidth will range from .6 dB on 7.100,
>> and 1.3 dB on 7.2 MHz.
>>
>> In both cases (HI-Q and CC coil) the SWR 2:1 band width is from 7.000 to
>> 7.280�a full 280 kHz !!!
>> This huge BW is because the driver is tuned for 1:1 SWR a full 100 kHz above
>> the max gain point.
>> As I said earlier, CC did this because the R is 50 Ohms�a perfect match to
>> coax and the Yagi can be �used� from the CW to SSB part of the band.
>>
>> However, if we tune the Yagi driver closer to the peak gain point, say at
>> 7.05 (which is also the peak F/B qrg), then the feedpoint impedance is only
>> 33 Ohms (we need a matching network).
>> The 2:1 SWR BW is now only 170 kHz�..from 7.000 to 7.170, so in-band
>> switching will be needed to go from CW to SSB.
>>
>> On both my 40m and 80m Yagis, I have a series of relay switched inductors
>> that will move the operating window across the band, so from CW to SSB, I am
>> almost always within a fraction of peak gain.
>> I have 4 segments on my 40m Yagi, and 18 band segments on my 80m Yagi. �All
>> relays switch automatically using the Station Master band decoder that
>> follows the radio.
>> Many have commented that relays can be un-reliable, and a maintenance
>> problem�well...my 80-40m Yagi has been in service for 18 yrs.
>>
>> Here is a paper I wrote explaining the 80m Yagi, that shows the in-band
>> switching system.
>> https://drive.google.com/file/d/1IksvND0LfeSBcH6j7UEWkyjkIvexDIgX/view
>> <https://drive.google.com/file/d/1IksvND0LfeSBcH6j7UEWkyjkIvexDIgX/view>
>>
>> 73, de steve ve6wz
>>
>>> On May 22, 2021, at 9:24 AM, Artek Manuals <Manuals@artekmanuals.com
>>> <mailto:Manuals@artekmanuals.com> <mailto:Manuals@artekmanuals.com
>>> <mailto:Manuals@artekmanuals.com>>> wrote:
>>>
>>> Steve et all
>>>
>>> So what is the "penalty" in DB (gain) for the using the increased bandwidth
>>> settings on this beam vs an optimized for gain approach? For the sake of
>>> discussion ignore the F/B degradation ...just raw dBd or dBi gain comparison
>>>
>>> Dave
>>> NR1DX
>>
>
> --
> Dave Manuals@ArtekManuals.com <mailto:Manuals@ArtekManuals.com>
> www.ArtekManuals.com <http://www.artekmanuals.com/>
>
>
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