Question about pass through cable and SWR Flip-Flop...

[johnsland8]

I just put up a 40m vertical wire antenna with 32 radials. I did some initial "rough" tuning at the antenna and my readings were:
7.000 = 1.9 @ 58 ohms
7.100 = 1.5 @ 50
7.200 = 1.3 @ 45
7.300 = 1.4 @ 48

I then ran my feed line through a flat pass through cable I'm trying out to go through a window into the house. I expected the readings to change slightly but this is what I got:
7.000 = 1.3 @ 50
7.100 = 1.2 @ 50
7.200 = 1.4 @ 55
7.300 = 1.7 @ 60

What I don't understand is why the readings basically flip-flopped from one end of the band to the other instead of just changing relatively? Any thoughts or input would be greatly appreciated.

Thank you!

 

[cmdrwill]

flat pass through cable

Sounds like your problem. May be a funkey "cable", AKA NOT 50 ohm cable. And even changing the length of good cable can change SWR values.

 

[a417]

I wouldn't trust those things.

Tune it AFTER you put those flat pass doohickeys in, that way you're not changing the infrastructure any more.

 

[WA0CBW]

The "flat" pass thru cables are not coaxial and have a different impedance.

 

[johnsland8]

I wouldn't trust those things.

I don't think I do either. I ordered them online and when they showed up I was immediately concerned. But, they were cheap and figured I'd give them a try.

Tune it AFTER you put those flat pass doohickeys in, that way you're not changing the infrastructure any more.

This was my thought too but what confuses me is that, from what I understand, if the SWR is lower on the low end of the band, the antenna is too long and if the SWR is lower on the high end of the band, the antenna is too short. So, if I use the pass through, the antenna needs to be shorter but if I don't, it needs to be longer. I guess I just expected the SWR to change but not change which way it was tuned. If that makes sense?

As you can tell, I'm new to this. I currently live in a rental home so I was looking for an easy way to get inside without any home alterations. I think I'm going to have to pursue other options.

Thank you for the input.

The "flat" pass thru cables are not coaxial and have a different impedance.

Didn't really think about that but it makes sense they couldn't be coaxial and be that thin.

Sounds like your problem. May be a funkey "cable"

I think you're right. I just wish I understood better what was happening to cause the SWR to change ends of the band.

 

[WA0CBW]

The SWR is indicating the impedance match (mis-match?), not the antenna resonance. You can have a resonant antenna but the feed point impedance can be something other than 50 ohms (and usually is).

 

[prcguy]

It could also be you have lots of common mode currents on the coax and the coax is the major counterpoise of the antenna system. In that case lengthening the feedline as in adding a small chunk of cable can have the same effect as lengthening the antenna and retuning it. Since you were a little short on the antenna to start with and now it appears the antenna has gained length, this could be a real possibility.

I would at least insert an effective common mode choke in the feedline right at the feedpoint or about where the radials end and the coax extends past them. How long are your radials?

 

[johnsland8]

How long are your radials?

Of the 32 radials, 18 of them are 33 feet. The other 14 vary from 24-32 feet (working around obstacles).

 

[johnsland8]

I would at least insert an effective common mode choke in the feedline right at the feedpoint or about where the radials end and the coax extends past them.

If you could please indulge me on this? I'm not sure how to do this. I have seen the ferrite chokes that will snap on the exterior of the feedline but when I did a search for a common mode choke, they essentially look like wire wrapped around a toroid. How would I incorporate this? Sorry for all the silly questions...

 

[prcguy]

A common mode choke at your feedpoint will keep RF off the feedline and will help isolate your antenna from changes in the feedline, etc.

A very common home made 1:1 choke balun would be 9 turns of RG-58 size coax around an FT-240-31 toroid. This should be good for at least several hundred watts over 80 through 10m and if you need full legal limit use RG-142 Teflon coax. If you only need 40m then 12 turns would give better performance across 80 through 17m giving up some on 10m. You can also use an FT-240-43 core with 12 turns to cover 40 through 12m real nice or 17 turns will give better choking over 60 through 30m.

You can simply wind the turns like this counting each time the coax passes through the hole as one turn: https://palomar-engineers.com/wp-content/uploads/20131222_125533.jpg or many people agree its better to cross over the turns mid way through so the input and output are opposite each other like this, and never mind this pic shows line cord instead of coax: https://owenduffy.net/blog/wp-content/uploads/2015/05/FT240-43-11t.jpg

You can also use a giant 31 mix snap on core with 1" ID hole and wrap 6 or 8 turns around that. I've been buying most of my 31 mix ferrite from this guy who seems to have the best prices around. Fair-Rite Mix 31 Ferrites - Pro Audio Engineering

Or you could just buy the best 1:1 choke balun I know of for any price here, but its much more expensive and will cover way more bands than you need with your 40m vertical. CMC-130-3K 1-30MHz

If you could please indulge me on this? I'm not sure how to do this. I have seen the ferrite chokes that will snap on the exterior of the feedline but when I did a search for a common mode choke, they essentially look like wire wrapped around a toroid. How would I incorporate this? Sorry for all the silly questions...

 

[WA8ZTZ]

Actually, your vertical antenna installation with 32 radials sounds quite nice.
Don't obsess trying to get a perfect 1:1 across the band, ain't gonna happen.
If anything, spend your time and money making sure the grounding and lightning
protection is satisfactory.

 

[AC9BX]

It sounds like the flat cable is both lossy, as I would expect, and is transforming impedance, which I would not expect to see very much change unless it's rather long or WAY off from 50 Ohms. At 7MHz a foot of mismatched line won't make a dramatic change.
Lossy line is great at making your system seem better than it really is.
"My antenna has been up there for 30 years and it keeps getting better." - nope, your feed line is getting worse.
Common mode current could certainly be an issue as PRC says. If you have a lot the coax shield could indeed be acting as a counterpoise, which you don't want, and it's getting disrupted by this flat line.
I assume you have some fashion of analyzer since you posted impedance values. If the gear offers it I would check for the complex impedance value. Is that 50 Ohms appearing purely resistive? It shouldn't be for a vertical. You ought to see a fairly low impedance, around 25 Ohms, real and some imaginary value. Say, total Z = 30 Ohms, 24 +j6, or some such. You may want to know if the antenna is resonant, where the imaginary j value is zero. It will be more efficient there. But will likely not be 50 Ohms at that point. That's okay. That's what matching is for, if needed. If it's close enough and you have good coax the mismatch loss will be of little concern. Don't obsess over SWR. If you've trimmed the height looking for 50 Ohms you are likely pretty far from resonance. This is all under ideal conditions and any number of things can push this all out of wack.
You need to measure the antenna at the feed point. That's tough with a vertical because you standing there messes with the measurements. Ideally you need a cable some multiple of a 1/2 wavelength. That moves the feed point impedance to the end of the coax making the coax invisible, aside from its loss.

 

[W5lz]

The only thing I would add to AC9BX's post is to make that 1/2 wave length of coax an -electrical- half wave length, takes VF into account and can make quite a difference sometimes.

 

[W5lz]

Oh, and as far as the original 'numbers' are concerned, there's not enough difference to make any difference at all.