bug catcher shunt coil problem

[mr_fx]

so I got my bug catcher built, setup for 40-6... about 32 turns around a 3.5" form, it's around 6" long, then the coil to pulled off and zip tied to a 3/4" fiber glass rod that has 3/8-24 fitting on either end, 52" whip on the top and a 3.5" copper master mobile mast with a 6" section of mini8 braid going from the mast to the base of the coil

the shunt coil was some 14 gauge magnet wire, coiled around a 1.06" form...

I started by going to to the point with the lowest reactance in the 40m band... then I hooked up the shunt coil... and adjusted it for a near 50ohm load, nice 1.2:1 match at 7.125Mhz or so, problem is that NOW by the time we get to 7.250Mhz I see around 10:1 SWR... if I unhook the shunt coil it drops back down to 4.0:1 and stays around there for the entire band

6m works great on the 17m tap, usable match

10m works fine on the top (shorting all the coil), usable match

11m works fine on the top, usable match

12m, almost at the top of the coil, usable match

15m, 2 turns down from 12m, usable match

17m, about 2 turns down from 15m, usable match

20m 2 or 3 turns down from 17m, usable match

40m is around 6 to 8 turns up from the bottom

 

[W5LZ]

Leave the 'shunt'/matching coil alone and move the tap on the loading coil for a change in frequency. It shouldn't be much of a change, maybe from one side of the loading coil to the other.
- 'Doc

 

[mr_fx]

so adjust the coil to were it works best then move the tap on the antennas coil?

 

[W5LZ]

Think of it like this. You are doing two things, adjusting for resonance with the loading coil, then matching impedances with the matching coil. If you move frequency then the part that's 'off' is the resonance (adjusted with the tap on the loading coil). That shouldn't be a huge move, maybe just to the other side of that loading coil (180 degrees around). It's one of those try it and see thingys
Then, if the impedance match isn't okay, do the same with the matching coil's tap. That one will be a very small move, probably just a few degrees around, sort of. I doubt if you'd have to change the matching coil at all.
Once 'set', that matching coil shouldn't have to be changed unless there's a big change, as in bands, not frequency on the same band. I also wouldn't be looking for a '1:1' SWR, anything around 1,5:1 is about as good as it gets.
This sort of procedure is how the screwdriver antennas are adjusted too, not just the bug-catcher.
Another little thingy with loaded mobile antennas is that it's overall length with the loading coil completely by-passed is the highest frequency of use. Your antenna is too long for 6 meters, so expect it not to be the bestest in the world there. It can certainly be made to 'work' there though...
- 'Doc

 

[mr_fx]

well it's working great now, took about an hour to find all the points on coil

adjusted the shunt coil to around 50 ohm on 40... then checked 30, 20, 17, 15, 12, 11, 10, nice flat SWR and more importantly all bands had points with 0 reactance,

40m needed about 12 tap points to cover 7.0 - 7.3 with 1.6:1 or less swr
30m needed 1 tap
20m needed 2 tap points
17m needed 2
15m needed 1
12m needed 1
11m and 10m both work by shorting out the coil...

NOTE: 6m works around the 17m tap, basically it sees it as an electrical 3/4 wave, but physically it is closer to a 1/2 wave, my guess is it performs about like I would imagine a base loaded 30" whip would on 6m... not terrible, but not too impressive either...

it's pretty darn ugly, but it works

also the noise level used to drown out the lesser stations... constant s8-s9 with the car running, after adding the shunt coil it's usually around s0, not only can I hear guys on my morning net I have never heard before, BUT I can hear them earlier than usual

future plans include a cap hat so I can work more stations and maybe even work 80m mobile... with a 52" whip I can currently tune down to the mid 6Mhz to upper 5mhz range... the draw back is I will have to use a non fundemental on upper bands, like 17-15-12-11-10 and I most likely loose 6m...

I dont really care about 6m on this antenna as I plan to strip my workman hamstick wanna be antenna and convert it in a mini bug catcher specifically for 20m and up witha cap hat

for 6m I plan to run a full wave horizontal loop