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Roushon 7th January 2013 04:07 AM

J-pole phenomena
 
I just built a J-pole antenna for my tuner. It was for 99MHz as I calculated
to be the best for me and used the calculation from this page.

J POLE ANTENNA DESIGN CALCULATOR by K4ABT

Tested yesteday (Sunday morning) using a 75Ohm cable and was very happy that all stations were coming in full singal (3 leds ON) except 107.1MHz (two leds ON). But I listened to this station most times so rebuilt the antenna for 100MHz (cutting 20mm from the long and 6mm from the short pole) in the hope that I wil get 107.1MHz also full signal. To my surprise now (Sunday night, Monday) three more stations are not coming in full signal (that is only two leds ON). What could be the reason? When I tested first (99MHz antenna)
I did not build the coil (5in diam) as mentioned in the page, but had the coil when I tested in 100MHz antenna. Not sure if this could be the reason or
on Sunday morning the signals were strong? I already fixed the antenna on my balcony, so not able to test different alternatives easily. Will do so on next sunday, before that I request for your suggestions.

omishra 7th January 2013 05:13 AM

Did you slide the antenna wires to adjust best position before fixing? You have to adjust your feed points to practically best gain position. Also you have create proper impedance matching loop in coaxial wire. This will save some signal loss.

Roushon 7th January 2013 06:08 AM

Quote:

Originally Posted by omishra (Post 3314287)
Did you slide the antenna wires to adjust best position before fixing? You have to adjust your feed points to practically best gain position. Also you have create proper impedance matching loop in coaxial wire. This will save some signal loss.

Thanks for the quick response. I did not change the feed points in the new antenna, also the design in the
page gave a fixed distant feed points. I will try the variations. The 5in 4 turn coaxial(75Ohm) loop coil was basically to avoid RF I think. Will try without it also again.

Regards
Roushon.

Roushon 7th January 2013 10:28 AM

Bypassing coil
 
Yes, if I bypass the coil then the signal improves in the sense that I get two more stations in full signal. Will try other possibilities...

DF96 7th January 2013 12:11 PM

The J-pole is essentially a narrowband end-fed halfwave, with a quarterwave matching section. It suffers from two problems:
1. as the (internal) feedpoint is high impedance it can be easily affected by nearby objects
2. without a balun it relies to some extent on feeder pickup

I would not regard it as a suitable FM broadcast antenna. The folded dipole is better as it has wider bandwidth and is less affected by objects. With a balun it avoids feeder pickup.

Roushon 8th January 2013 04:21 AM

Quote:

Originally Posted by DF96 (Post 3314632)
The J-pole is essentially a narrowband end-fed halfwave, with a quarterwave matching section. It suffers from two problems:
1. as the (internal) feedpoint is high impedance it can be easily affected by nearby objects
2. without a balun it relies to some extent on feeder pickup

I would not regard it as a suitable FM broadcast antenna. The folded dipole is better as it has wider bandwidth and is less affected by objects. With a balun it avoids feeder pickup.

Thanks for your suggestion. As I do not have space on my balcony yagi type antenna will be difficult to install. Also there is another problem of direction to which the broadcast is coming from.

Regards
Roushon

DF96 8th January 2013 09:48 AM

I didn't say Yagi, I said folded dipole. The dipole, whether folded or not, has exactly the same polar diagram as the J-pole. It is shorter and thicker. Instead of being mounted at the end it is mounted in the middle, which may be a disadvantage if you need vertical polarisation. For reception of local vertically polarised signals the 'ground plane' may be sufficient: quarterwave antenna over a set of four quarterwave counterpoises. For local broadcast reception you may be able to get away with just two counterpoises.

The main problem with FM reception is antenna bandwidth. You need 20%, yet most simple wire antennas are more like 2%.

Roushon 8th January 2013 10:42 AM

Quote:

Originally Posted by DF96 (Post 3316182)
I didn't say Yagi, I said folded dipole. The dipole, whether folded or not, has exactly the same polar diagram as the J-pole. It is shorter and thicker. Instead of being mounted at the end it is mounted in the middle, which may be a disadvantage if you need vertical polarisation. For reception of local vertically polarised signals the 'ground plane' may be sufficient: quarterwave antenna over a set of four quarterwave counterpoises. For local broadcast reception you may be able to get away with just two counterpoises.

The main problem with FM reception is antenna bandwidth. You need 20%, yet most simple wire antennas are more like 2%.

I am on my learning curve. Finding the quarterwave very interesting. Some questions: do the counterpoises need to hang in the air from the ground plane or it supports the whole structure? Also do the angle of counterpoises to the main pole matters? And what should be their lengths? Found out the main pole should be (75/frequency) meters. A bit confused after seeing many different pictures in the net?

Regards
Roushon

Roushon 8th January 2013 11:03 AM

One more question: how does one usually increase the bandwidth of an antenna? I mean what is the standard procedure: like thickness of the materials! etc.

Roushon.

DF96 8th January 2013 01:44 PM

The simplest way to increase bandwidth is to make it wider or thicker. That is why a folded dipole has wider bandwidth than a normal dipole.

The angle of counterpoises of a 'ground plane' affect feed impedance. Assuming the main part is vertically up, then horizontal counterposes give about 35 ohms, vertical (below) give about 73 ohms, in between angles give in between impedance.


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