VHF receive problems - Jpole - APRS
I may not be able to make tonight's Elmer night, so I'll throw this out here on the forum.
I recently installed an Arrow Open-Stub J-Pole antenna in my attic. Seems like there's not much metal around it to interfere. It's nice and high in the second story attic. I've got a short run (25') of LMR-400 coax to my radio in my office. SWR is pretty flat at 1.3 or so on VHF.
I have no problems using this antenna for normal stuff, like nets. Seems to receive clearly, TX is good by reports.
What I have discovered however, is that my receive seems weak. I learned this by using APRS on the same radio. I'm not able to receive APRS packets. I can transmit packets OK; they show up on aprs.fi. Packet Winlink has the same issue, no connect with gateways, but maybe not hearing them.
When I switch the identical radio setup to a roll-up monoband jople I built, I get all sorts of packet activity. I can even get reception using my SignalStick on the HT. But swapping back to the Jpole gives me no packet reception. All of these antennas are indoors, within a few feet of each other.
So: VHF antenna in attic seems to work well on repeaters, but not on APRS reception. I am guessing that reception is not great, and that the repeaters just have more power so I've been OK.
Anyone have any other ideas? I'm going to pull it down from the attic and try it right next to my rollup. Maybe there's more metal than I suspect up in the attic.
Thanks in advance.
Tom - KF0AER
If you can give some more detail on the mounting arrangement of the Arrow Open Stub antenna, or even attach a quick photo of how it's installed may help. You mentioned concern about keeping the antenna away from metallic objects, which is a valid concern, but keep in mind that non metallic object can affect how an antenna works also. Non metallic object can have affects on the velocity factor of the antenna conductors and this often will change then affective tuned frequency of the antenna system.
What radio are you using in this station setup?
First off on the antennas, even though many people call the Arrow antenna you are referencing a "J-Pole", it is not the same type of antenna design as what you are calling your mono band role up j-pole. I have not used one of the Arrow open stub antennas before, but just based on antenna theory, it is pretty safe to assume that it would be less efficient than the typical J-Pole. I would not expect a big difference though (if the typical J-Pole, being a Half wave end fed radiator with a shorted 1/4 wave matching section). I have seen a lot of people give good reviews of the Arrow antenna. It's one thing to be able to get into a repeater with ease, than trying to pick lots of APRS transmissions which are often much weaker in comparison to what you would receive from a repeater
Compared to a repeater output an APRS transmission starts a lot weaker in many cases. Keep in mind a repeater often starts with 100 watts or more. This goes to very low loss feed line, Usually to a high gain antenna, often mounted very high above ground. And here in Colorado at high altitude with a view well above the Metro Area. All of this together makes for a very strong signal for you to receive.
With APRS at least for the original source packets a mobile radio is starting with less power, and packets coming from HT's start with a tiny amount of power compared to a repeater. This then feeds usually into some sort of compromised handheld or mobile antenna installation, low to the ground, and blocked by surrounding terrain. Be mindful of these deference's in what you can guess about the station that is transmitting a certain type of signal when trying to compare two different antennas together.
FM can also be tricky when trying to compare different signal apparent strength when just listening to the received audio. Often with typical FM receiver there is no incoming RF signal strength meter. Due to what is typically called capture effect for FM radio listening to the demodulated audio quality can't really tell you much about signal strength. With FM radio you will have a noise in the demodulated audio from a weak RF signal untill the signal strength increases enough for the FM receiver to get proper modulation. Once that point is reached you can greatly increase the strength of the RF signal and you will hear very little change in the demodulated audio quality.
In your case comparing received APRS to Repater signals a small difference in antenna gain could make a big difference. With one antenna you might have just enough gain to receive the weak APRS signal so that it can properly be detected by the FM reviver and be properly demodulated. It may be right on the edge of what the receiver can pickup, but it might be enough. Even though the signal from the repeater might be delivering 10 times or even 1000 times, or more RF energy at your receiver, the demodulated FM audio is not really going to sound very different.
I think in your case your dealing with weak signals with APRS in your case and you are starting to see just how big of a difference antenna system can really make when it comes to weak signals.
Another point of comparison I can give you from my experience. I did a bunch of comparisons with a Diamond Discone D130J antenna out at my house in Strasburg about 25 Miles east of Denver. I started with it mounted to a 10 foot pole out side in my backyard just for initial testing. I had about 50' of LMR 400 to get to the radio in the house. I also compared this to mounted in my attic inside the house where it was about 16 feet above ground and used about the same length of feedline. With the Antenna outside I could get into the PRA repeater will with 5 Watts of output from an IC-9700. I never really tried running lower power to see where the received audio quality reports would start to drop off. Keep in mind that on a 10 foot pole the antenna still did not have line of site over my House to the mountains. When I relocated the antenna inside my attic at a high location I had to increase my transmitter output power up to 40-50 watts, depending on the day, to get the same audio quality report back from some one listening to the repeater. With that same antenna in my attic I could not even open the repeater squelch with 5 watts.
If you find that the J-Pole type antenna you made works that much better, but you really want dual band you could always make separate ones for 2m and 70cm and use a simple switch or a duplexer.
I have been using a 2 meter 4 element yagi in my attic also some. Since I am so far east of the metro area the, Yagi was a wide enough directional coverage that I can just point it west in the attic and get good coverage of most of the metro area and all the repeaters I am interested in. Sensitivity drops off down towards CO springs and North to Boulder/Longmont. Can't pickup much weak signal stuff, but it works for the repeaters.
If you are interested in building some thing else that is relatively easy to put together that give a little gain and some directionality take a look online and search for Moxon Antennas. A lot of people out there build them for the HF bands, but they can be very easy to build for 2 meter and 70cm.
Hope this helps and does not confuse the issue more. Let me know what other questions this raises.