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.
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.
Last edited:
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.
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.
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
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%.
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
Last edited:
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.
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.
Jpole tuning
I did some tuning with the J-pole. Sliding the feed point and placing it on a different location etc. Now there are lot more stations I can tune to than I thought exists. But there is one puzzling issue. At few frequencies I hear two stations overlapping. Is it possible to separate them?
Will make one dipole also soon.
Roushon
I did some tuning with the J-pole. Sliding the feed point and placing it on a different location etc. Now there are lot more stations I can tune to than I thought exists. But there is one puzzling issue. At few frequencies I hear two stations overlapping. Is it possible to separate them?
Will make one dipole also soon.
Roushon
If the two stations are actually on the same frequency then only a direction antenna (e.g. Yagi) can separate them. If they are on nearby frequencies then a better tuner with narrower IF filters may be able to separate them. If one of them is not really there but created by intermodulation then either a better tuner with a more linear front-end or adding an attenuator to weaken the signal will help.
Thanks again for your responses. I believe it is the first possibility, will have to try with a different antenna.
Regards
Roushon.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.