bug catcher 10m reactance problems

[mr_fx]

well I built a 2nd bug catcher, used a 20 meter ham stick gutted it, slid the braid of some old RG8U over it and built a 36 turn 3.6" inside diameter coil (2.4mm wire, about the same in spacing...

with the coil shorted out the antenna seems to resonate around 30Mhz or so

this means I have to run the tap on the 2nd turn of the coil (on the top) problem is that I see about 20-30 reactance on the MFJ-259... my other antenna ( nearly a foot taller) while TECHNICALLY a little too tall for 10m works quite well actually and does not require ANY coil and has much lower reactance (less than 5)

the other antenna is also much more broad banded than that smaller model, they both do pretty good below 10m, in fact in double blind tests people could not tell the difference 9 times out of 10 they gave them the same report... keep in mine they are on the same car, but on opposite sides of the vehicle

 

[W5LZ]

The only 'trick' in any of this sort of thingy is finding where the thing is resonant and adjusting from there. When you find the resonance point, then you can worry about matching impedance (SWR). That can get sort of 'hairy', it isn't as easy as most people think. The two things, impedance matching and resonance are entirely separate thingys, resonance doesn't mean 50 ohms impedance by any means. In fact, it's almost a miracle if a resonant antenna has an input impedance of 50 ohms without a matching device of some kind.
The biggy is resonance. THEN you worry about impedance matching (SWR).
Not exactly how it's commonly done, huh?
- 'Doc

 

[mr_fx]

ok I was playing around with the one on the drivers side (the shorter of the 2 antennas) and I put it on he 20m tap point... could not get a match it was a little high...

then I switch back to the passenger side antenna that was already on 20m... but now it did not have a match, it was almost 4:1

THEN I touched the coil on the drivers side antenna and the match on the passenger side antenna changed...

they are both shunt fed at the base...

I also noticed that this was not as noticable of an issue if the bands used are different on either antenna

 

[mr_fx]

I guess I am trying to figure out if the 2nd antenna is exciting through the shunt coils or if if is coupling, like how a beam works...

that being said, this might explain why 20m has seemed like it was opening late and closing early lately

I might have been dumping on the other antenna by mistake...

Would it help if I kept the antennas on non related bands?

My main antenna pretty much stays on 14.225-14.235 (the 9.33 foot beast thing)

the other one spends most of it's time 15m and 10m (the 7.5 foot mini stick)

I mean the fact that I can not find a resonate tap on 10m (a tap without reactance) on the 7.5 foot antenna tells me that 10m on one antenna is interacting with the 9.33 foot antenna while it's on 20m...

both antennas are near each other, about 7 or 8 feet apart

 

[W5LZ]

The closer any two antennas (near each other) are in resonance the more they will affect each other. Changing one of those antennas to a non-harmonically related band will certainly make for less mutual affects.
If the positioning of those two antennas can be manipulated that mutual 'affect' can be a constructive type of thing, like a beam. There are a lot of 'catches' to that, right distance apart, does it 'point' in the desired direction, and so on. Except for very unusual conditions/situation, that's not something that's all that handy to have. Or, it may be handy to have, but getting it may not be all that easy/practical?
- 'Doc

 

[Captain Kilowatt]

ok I was playing around with the one on the drivers side (the shorter of the 2 antennas) and I put it on he 20m tap point... could not get a match it was a little high...

then I switch back to the passenger side antenna that was already on 20m... but now it did not have a match, it was almost 4:1

THEN I touched the coil on the drivers side antenna and the match on the passenger side antenna changed...

they are both shunt fed at the base...

I also noticed that this was not as noticable of an issue if the bands used are different on either antenna

Congratulations. You just found out what happens when a resonant object is brought into the near field of an antenna. It's called coupling and detuning. Different bands are not so bad but the same band can be disastrous.

 

[mr_fx]

Doc I am trying to avoid the coupling

I know that 20m and 10m are harmonically related, in that a 20m antenna is also a 1/2 wave on 10m... So if it was fed in the center it would be a 1/2 wave 10 meter dipole... well electrically anyways...

My goal WAS to have 10 bug catchers and to let the smaller one be tuned to 15, 12, 11, or 10, with the taller one running 17, 20, occasionally even running 40m

but it seems these 2 antennas want to have inappropriate realations with each other

 

[mr_fx]

Congratulations. You just found out what happens when a resonant object is brought into the near field of an antenna. It's called coupling and detuning. Different bands are not so bad but the same band can be disastrous.

bringing me to my next question... if I tune an antenna (antenna A) and then ADD an antenna ( Antenna B) and antenna A keep it's same match that it had before adding antenna B then is it safe to assume that they are not coupling, or at the very least the coupling is minimal?

 

[W5LZ]

Good question, and I'd have to think that if there's no change in antenna 'A' then there was minimal reaction between them. But that goes back to just how close they are together, the frequency spread, and who knows what else. I would think that there would be minimal changes in 'A', but that there would probably be -some- change.
I'm probably reading your intent wrong, but why would you want 10 antennas? Changing the loading coil tap on one would eliminate probably 8 of them (how it's normally done). Pick the total length and size of loading coil correctly and you could do it with just one antenna. How would you even mount ten of those 'buggers' on one vehicle? I'm misreading that, right?
- 'Doc

 

[Captain Kilowatt]

bringing me to my next question... if I tune an antenna (antenna A) and then ADD an antenna ( Antenna B) and antenna A keep it's same match that it had before adding antenna B then is it safe to assume that they are not coupling, or at the very least the coupling is minimal?

Yes it is safe to assume that. This is the reason that you have to be careful with guy wire lengths. If they are a resonant length on one or more bands then they can easily couple to the antenna and affect the SWR and radiation pattern.

 

[mr_fx]

problem solved...

went to a 52" top whip (had to boar out the ferrule as this whip as a little thicker , it was nearly 10" longer than the existing whip that came with the antenna)


It still requires a minor ammount of loading on 10m (it's a hair short) but now I can get the reactance on 28.325 to 2 and a near perfect 50 ohm load...

it does not seem to cause any issues with the 20 meter antenna

turns out that Jack (on the east coast) has been having trouble hearing the midwest boys all last week... it was just so happened that as the sun was going crazy, I chose that time to test a new antenna...

so as long as the antennas are not tuned both to 20m they should play nice together...