3
$\begingroup$

I live in a fairly sheltered valley but there are several hill-top VHF repeaters that I can receive very well. However the one repeater in particular I want to transmit to, I can't reach well. I know my signal is reaching it, because it will auto-respond its call sign when I transmit, but I get reports that my signal is unintelligible.

I've using a BaoFeng F9V2+ with a Nagoya NA-771 antenna and I've also tried a Slim Jim but have not been successful at getting an intelligible signal through.

My question is, would using a copper-pipe J-Pole be any more likely to be successful than the Slim Jim or are they pretty much equivalent?

Improve this question
$\endgroup$
1
  • $\begingroup$ Were you able to reach the repeater with the Nagoya antenna? Were you able to verify the slim jim antenna with an analyzer? Kevin Reid's answer regarding a Yagi is spot-on, but if you are up for a little experimenting, a simple dipole properly tuned and fed with low loss coax (LMR-400, for example) should give noticeably better results than the handheld antenna. How far away is the repeater? Do you have line of sight? $\endgroup$
    – Chris K8NVH
    Aug 20, 2018 at 2:47

4 Answers 4

Reset to default
5
$\begingroup$

The slim jim antenna has no more or less gain than a J pole antenna - despite many Internet claims to the contrary. Any perceived difference in gain is likely due to CMC (common mode current) since both antennas promote CMC on the feedline.

There is an argument to be made that a larger conductor improves efficiency and therefore gain. But for antennas that are at least 1/4 wavelength or longer, as in this case, the effect will typically be in the sub dB range.

You may also find references to the TOA (take off angle) being different between the J pole and the slim jim. Antenna range measurement and simulations show this to not be the case. This apparently came about when Fred Judd, G2BCX stated that the slim jim has a better TOA than a (5/8 wave) ground plane. This comparison was later morphed on the Internet to be a comparison between the slim jim and a J pole. Any such perceived difference in TOA is most likely attributable to CMC.

Improve this answer
$\endgroup$
4
  • $\begingroup$ There is a difference in take-off angle between the two antennas. A slim jim has a lower take-off angle than a J-pole. Whether this is superior or not depends entirely on where the transmitter is and where the repeater or distant station is. $\endgroup$
    – Jim MacKenzie VE5EV
    Aug 20, 2018 at 4:19
  • 2
    $\begingroup$ @JimMacKenzieVE5EV Unfortunately, this is not true. Antenna range measurements of the two antennas, with current baluns at the feedpoint, show equal take off angles (within 1 dB). Any perceived difference in TOA is likely due to common mode currents. $\endgroup$
    – Glenn W9IQ
    Aug 20, 2018 at 9:53
  • 1
    $\begingroup$ @JimMacKenzieVE5EV I added more about this to my answer. $\endgroup$
    – Glenn W9IQ
    Aug 20, 2018 at 10:04
  • $\begingroup$ Thanks for your help on my question. I'm gonna try hanging the Slim Jim higher and if that doesn't work, I'll try a Yagi. $\endgroup$
    – Brad McCarty
    Aug 20, 2018 at 14:57
4
$\begingroup$

I'm not familiar with empirical results for these particular antenna designs, but in general, an antenna constructed out of thicker conductors will have a wider bandwidth. That is, its impedance will be more consistent across the band.

Theoretically, this is an improvement because you will get a better impedance match across the band, including the input frequency of your target repeater. However, the actual effect is likely to be small (unless you have a particularly long or lossy feed line between your radio and antenna), and it might even happen to be worse if the new antenna is not well tuned.

Unless it's really cheap and easy for you to build a copper J-pole, consider building or buying a Yagi type antenna — even a 3-element one — and aiming it at the repeater. This will improve your signal strength.

Improve this answer
$\endgroup$
1
  • $\begingroup$ Thanks @KevnReid regarding the Yagi, I'm going to try that. It'll give me a chance to build my own and learn more. $\endgroup$
    – Brad McCarty
    Aug 20, 2018 at 14:54
4
$\begingroup$

In a situation like this, gain helps a bit, but usually height helps more. So whichever antenna you can get higher will probably work best.

Also there are some odd propagation things around mountains, check out knife edge propagation and fresnel zones. Without getting into technical details, you can take advantage of constructive and destructive interference of multipath propagation by moving a small antenna within an area very roughly less than 1/2 wavelength to find the spot where you get the maximum reception. This spot will also give you the clearest signal when you transmit.

Improve this answer
$\endgroup$
2
  • $\begingroup$ I haven't been able to get the Slim Jim up high yet, I'll try hanging it in the trees as the first easy step. Thanks $\endgroup$
    – Brad McCarty
    Aug 20, 2018 at 14:56
  • $\begingroup$ When you hang it in the tree, try to adjust it so the metal doesn't touch the tree at all. $\endgroup$
    – user10489
    Aug 21, 2018 at 11:28
0
$\begingroup$

Line of sight is key when dealing with FM and considering you are using a HHT radio the wattage is probably 5-8watts max. Right? A J Pole has no real gain and I fear the loss would be worse than using a simple SMA antenna.

Improve this answer
$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .