[hpsdr] VNA kit -- parts for reflection bridge for use with Hermes

[David McQuate]

QST:  all those interested in a reflection bridge for use with Hermes.  
There are currently 56 kits requested.

The design being considered is one by Sam Wetterlin, who has built 
several different reflection bridges, and has posted their printed 
circuit designs on his web site.  http://www.wetterlin.org/sam/      
(look down about 2/3 of the page)

Initially it seemed that only the MiniCircuits balun transformer was 
needed, but many have expressed interest in having other parts added to 
the "kit", especially the small diameter coax, the ferrite beads and the 
printed circuit boards.

The PCB layout for the "3 bead balun" reflection bridge requires 
shortening the projections on a connector.  I would like to modify the 
PCB design so no connector modification would be required, and so that 
the board and connector really match well.  One respondent suggested a 
connector made in China, and available in quantities of 100 on ebay at 
very low cost---

http://www.ebay.com/itm/100X-SMA-Female-Jack-Edge-Mount-PCB-PC-Board-Receptacle-Adapter-Connectors-New-/290691281049?pt=LH_DefaultDomain_0&hash=item43ae8cf099

I have received a dimensioned drawing of this connector from the seller.  The PCB design could very easily be modified to accomodate this connector.

Are there any objections to using this connector?

Digikey part numbers J716-ND   and   991-1047-ND   are also possibilities.
The first is much wider than the either the second or the ebay part, so the PCB design would need greater changes.

In addition, Sam has made suggestions for changes that I would like to 
include that would increase the bridge's performance.

Another suggestion is to combine the two small circuit boards of Sam's 
design into one longer board, eliminating the connecting brass (shim 
stock) strip.  This would increase the PCB area, and therefore the cost, 
but it would also simplify construction.

I would want to build at least one bridge using a modified PCB design 
before building buying for more than 50, but this would require a few 
weeks to accomplish.  Another reason to build one is to be sure of the 
length of coax required to thread it 3 times through each of the 3 
ferrite cores.  If I'm going to build one, I would need to purchase at 
least 20 of the MiniCircuits balun transformers (their minimum 
quantity).  On the other hand, if I know that more than 50 will be 
wanted, it would make sense to just buy 100 at the outset, since there's 
a substantial price break.  (20 @ 2.19 each, 50 @ 1.49 each, or 100 @ 
1.19 each)

It has also been pointed out to me that thewireman.com has RG-178 at 
0.55 per foot in 100 foot quantity, about half the price compared with 
Digikey.  I think an 18-inch length would be more than enough--though I 
would certainly want to try it before chopping up a lot of coax!  
Similar consideration here regarding quantity.  100 feet would probably 
be enough for 66 bridges.

Now there may be some who would not want to wait that long, and who 
would prefer to "roll their own", purchasing only the MiniCircuits 
transformer from the group buy.
Dick K9IVB is forging ahead independently & has posted parts lists for 
all four of Wetterlin designs.  See near the bottom of: 
http://k9ivb.net/hermes/

Another issue is the possibility of sizing the PCB so it fits (nicely) 
in some sort of (standard) enclosure (eg aluminum box).

All these considerations seem to push out the time frame.  Is that 
undesirable for some?

I thank you for your consideration & comments!
73,
Dave
WA8YWQ