[SCCC] Phased verticals

Michael Tope W4EF at dellroy.com
Sun Aug 18 03:44:58 EDT 2019 

Hi Bill,

It is not surprising that the 75 ohm quarter-wave transformer lines 
narrow your bandwidth. Those lines are only  1/4 wavelength at one 
particular frequency, so they start to introduce reactance as you move 
off their center-frequency. This is in addition to the reactance that 
the antenna elements introduces as you move off their center frequency. 
If the signs of those two reactances were opposite as a function of 
frequency, you could get a bandwidth enhancement, but unfortunately they 
are not.

A quality broadband 2:1 transformer, on the other hand, will provide a 
near exact 2:1 transformation across a wide frequency range, so that 
only the antenna element reactance limits the bandwidth.

I think the only advantage of the 75 ohms lines is that you don't need 
the broadband transformer :-)

If you have not already done so, you may want to consider the impact of 
antenna element mutual coupling in your design. Unless your elements are 
electrically short (<< 1/4 wavelength), the input impedance of each 
element, when you are driving the array, will depart significantly from 
its standalone value (e.g. the input impedance of a resonant 1/4 wave 
vertical will no longer be 35 ohms resistive when that vertical element 
is in a driven array). This in turn can cause the phase shift from your 
phasing line to depart significantly from its electrical length (i.e. a 
quarter-wave delay-line only provides 90 degrees of phase shift under a 
very limited set of circumstances). Roy Lewallen W7EL lays it all out in 
this article:

https://www.eznec.com/Amateur/Articles/Simpfeed.pdf

Have fun.

73, Mike W4EF......................



On 8/13/2019 8:28 AM, Bill Shell - N6WS wrote:
> Bob, Marty, & Kurt,
>
> I don't mean to change the topic of Bob's thread here, but maybe you 
> guys can answer a question I had regarding the use of 75-Ohm coax 
> versus transformers for matching.  When I was putting together my 
> phased verticals for 160m, I first used quarter-wave sections of 
> 75-Ohm Coax for matching.  I found that the addition of the 'tuned' 
> section of 75-Ohm coax narrowed my usable 2:1 VSWR passband over what 
> I realized from a single antenna.  I had built a 2:1 transformer to 
> measure and test the 100-Ohm point to each antenna individually, so I 
> could run the test with 50-Ohm tester. After seeing how the additions 
> of the tuned sections of 75-Ohm cable narrowed the usable band for me 
> on 160m, I decided to try testing the 2:1 transformer and sections 
> 50-Ohm cable going to each antenna.  That configuration gave me the 
> most usable bandwidth on 160m.  The use of the 50-Ohm feed to the 
> antennas also allows the use of 50-Ohm cable for the phasing 
> delay-line injected at a 50-Ohm impedance point instead of the 100-Ohm 
> point when using the coaxial transformation method.
>
> After I went through two months of experimenting with how to feed my 
> 160m verticals, I wondered if I was missing something, or maybe not 
> measuring something correctly.  It seems so easy to use a transformer, 
> but most past and current magazine and Internet articles on phasing 
> verticals are still focused on the use of 75-Ohm coax for matching the 
> verticals.
>
> I understand my perceived advantages of a transformer versus coaxial 
> matching:
> - feeds to each antenna are just equal lengths of 50-Ohm cable, not 
> measured electrical lengths of 75-Ohm coax.
> - greater usable bandwidth due to not using tuned sections of 75-Ohm 
> coax.
> - better matching when using 50-Ohm coax for the delay-line.  (My VSWR 
> does not change appreciably when I switch N-S, E, or W.)
> But, what I don't understand are the advantages of using 75-Ohm coax 
> for matching.
>
> What I am really questioning is the advantages/disadvantages of my 
> using a transformer at the feed-point of the phased verticals. My 160m 
> verticals have worked out quite well using a transformer, but I am 
> always open to changing if there is a better solution.
>
> For Bob's question about 20m verticals, and future 40m verticals, the 
> transformer seems to me a logical solution.  You can just switch 
> antennas at the end of the 50-Ohm coaxes or use multi-band verticals.
>
> TU es 73,
> Bill
> N6WS
>
>
> On 8/13/2019 6:02 AM, W6PH via SCCC wrote:
>> Or an odd multiple of 1/4 wavelength 75 ohm cable, i.e. 3/4 or 5/4.  
>> The best theoretical gain is spacing the two verticals 5/8 wave for 
>> broadside firing.  The 1/4 wave cable length is too short because of 
>> velocity factor. So you would need two 3/4 wave 75 ohm cables to 
>> transform the 50 ohm to 100 ohms at the feed point.
>> Kurt W6PH
>> In a message dated 8/12/2019 7:52:13 PM Pacific Standard Time, 
>> n6vi at socal.rr.com writes:
>>
>> Hi, Bob.
>>
>> The most common way I have seen to match your transmitter to paralleled
>> 50-Ohm antennas is to feed each antenna with electrical 1/4 
>> wavelengths of
>> 75-Ohm coax to the Tee.  That will transform each feed point 
>> impedance to
>> 100 Ohms, and paralleling two of those gets you back to 50 Ohms.
>>
>> 73,
>>
>> Marty N6VI
>>
>>
>> -----Original Message-----
>> From: SCCC [mailto:sccc-bounces at contesting.com] On Behalf Of Bob Grubic
>> Sent: Monday, August 12, 2019 6:25 PM
>> To: Southern California Contest Club
>> Subject: [SCCC] Phased verticals
>>
>> I am considering using two 20-meter phased verticals for some portable
>> operations. (Later, two 40-meter verticals.)
>>
>> Keeping it simple at first, I'd run coax from my rig to a T-connector 
>> and
>> then through identical, same length pieces of coax to each vertical
>> (driving them in phase). I've seen this scenario several times 
>> online, but
>> the fact that two 50-ohm antennas would be in parallel at the 
>> T-connector
>> to give you a 25-ohm load (in effect), somehow bothers me.
>>
>> I've seen an article online suggesting using an UNUN to match the rig's
>> coax to the two legs feeding the antennas--essentially replacing the
>> T-connector (with a UN-22-25 from CWS). http://www.cwsbytemark.com/  I
>> can't find any other discussion of this particular subject online.
>>
>> Has anyone had any experience with this or thoughts on the subject? I'd
>> appreciate hearing about it.
>>
>> Thanks very much.
>>
>> 73,
>> Bob NC6Q
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