Discussion in 'Tuners' started by FauxHall, Jul 13, 2017.
ahh. There you go, then.
I did a quick Google-Fu and found this.
Of course this still does not mean that this is correct, it is just what I found.
Thanks, that link explains my observations.
Here's my latest configuration and the tuner signal strength meter on manual pegs high.
It's a dipole.
Vertical mounting a Dipole! simple. horizontal in all those damned manuals? for some reason the new dx crap digital.. (and many old tuners have good mpx and channel locking. etc. I did experiments with dipole configuration, 75ohm good cable etc. vertical was it! I set up a vertical outside and my internal dipole inside, no wire attaching both? Got more channels and stronger signal reception and reject on mute better, too.
I do believe we have a sponsor for antennas but you can gloogle up vertical fm antenna's.
Antenna's attract the waves. hooked up or not.. many can be simple absorbing ant. that capture deflected waves from buildings etc.
Just and idea I've been pondering.
Thanks for starting this thread. I bought two of these myself.Have connected one to my Mac. MR 78 all stations are coming in stronger have not had time to do much else
I'll have to check one out.
Here's what gets me:
You can have your feedpoint impedance be whatever you want; but how exactly are you solving the differences between a balanced and unbalanced signal? If there's no balun or even a choke; then effectively the shield of the coax is going to form part of the antenna. I mean...sure...you *can* stick the conductor and shield directly to coax at the right impedance and have it mostly work...but you still need some sort of transformation. Granted...this doesn't matter for RX as much as it does when you're pumping out 100 watts or more through it...but it's entirely possible the shield of that thing is acting as part of an antenna; which would not only detune it by lengthening it.
I'm not trying to say this is a bad antenna and that it won't produce some major improvement in reception...these are things the ham radio op in me wonders.
I'm almost tempted to get one just to tear it apart and see if they put any kind of choke or balun inside of it. I do like the fact it will do "circular" polarization (my understanding is it's not 100% true circular, just horizontal *and* vertical polarization)...though I have to wonder if making a folded dipole out of twinlead and matching the feedpoint for 300 ohms and sticking a 6:1 balun right at it would achieve the same properties. I think I see a new antenna analyzer in my future.
The last time I had a folded dipole out and set up 100% properly...was almost 3 years ago when I had a Japanese FM band folded dipole hooked in to a 9:1 balun I had handy in to a SDR. On a second floor balcony...for whatever reason; I was easily getting good FM stereo on stations well over 100 miles away.
If you go from 300 Ohms to 75 Ohms (folded dipole to 75 Ohm coaxial cable) why would you want anything other than 4 to 1 since most stereo receivers are 75 Ohms for the FM antenna input.
Of course the SDR dongles are 50 Ohms, but that is not really applicable. I am not sure why you would want 9 to 1 going into an SDR which is likely has a 50 Ohm impedance.
More of my OCDness.
Basic feed line theory 101.
They are being some what clever. Eight feet of coaxial cable with a velocity of propagation of about 80 percent (this is a very realistic value) will be a full wave length long. The impedance of the shield connection for the common mode current at the feed point of the antenna will be rather high, much like installing common mode RF chokes.
And BTW it is high at each end of the feed line (the antenna and at the receiver end). This is not necessarily a bad thing.
This should limit unwanted common mode current on the outside of shield of the coaxial cable. There is not any magic, only clever designs.
DewDude\s question can be partially illuminated by looking at J-pole design theory. It's an unbalanced design but one can adjust the feed impedance by sliding the contacts up or down the two poles.
The J-pole antenna is not a unbalanced antenna. Both sides of the feed line are equal potential with respect to ground. The point where the shield of the coaxial cable connects to the antenna is not at RF ground as it would have to be if the J-Pole antenna was an unbalanced antenna. The bottom of the J-Pole antenna is not at RF ground.
The J-pole antenna is an end fed (at a very high impedance point) antenna using a quarter wave transmission line impedance matching section. This does not make it an antenna with an unbalanced feed point, quite the opposite.
A J-Pole antenna works the best when it is fed with balanced feed line such as 300 Ohm twin lead and mounted on a non conducting support. The twin lead is connected a little farther up the shorted transmission line (the two pole as mentioned) to match the 300 Ohm twin lead.
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