Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KJ4RZZ, May 8, 2017.
Are you referring to the Mt Palomar repeater which is on top of a mountain at an elevation of 6000'?
You should be 5/7 both ways on 2m with 45W and slimjims at 30ft, and you are right to question it not working.
The two things that will wreck it, is terrain and crappy coax, so tell us about those. Ignore all the bleating about the slimjim antenna.
He has already stated his buddy has 1/4wl ground plane in the attic.
We don't know what "slim jim" means to him, e.g.. specific design, build quality, problems with feedline radiation, pattern skewing, etc.
If we are discussing a specific commercial antenna, we can have greater confidence our problem solving recommendations and it's performance. Baaah!
Sure we do. It's an end-fed dipole with unstated coax and terrain.
Lets hear about the coax and the terrain.
Well, my slim jim is the design here:
Built on 1/2" PVC pipe using enameled wire. It tests 1.0 SWR at 146mhz with my MFJ249-C.
I am running 40 feet of RG8X coax. Not great for 2M but it's what I had on hand. I don't use UHF, only 2 meter. My max power is around 65w, which is probably actually 30W due to coax loss, but if you add back the gain of the antenna, it gets me closer to 50w.
Terrain is FLAT, it's Florida. There is salt water directly between us, about 10 miles away (the Sunshine Skyway bridge). We both live in the suburbs, and downtown and tall buildings is not between us. We are both about 20' MSL with antennas at 30' AGL. Lots of trees between us.
I can easily hit any repeater with full quieting which falls within (LOS) range according to the curvature of the earth, even repeaters past my buddy from here.
Also, my buddy, using his 1/4 wave ground plane antenna, can hit the repeaters near me (also LOS) using only 5 watts, they are tall and above his radio horizon.
So you are saying we *should* be able to talk OVER the horizon 100% reliably at a 5/7? My understanding is that the signal gets attenuated very quickly once it hits radio horizon, and each of our radio horizons is at 8 miles from us, but we are 28 miles apart.
So thats 10 miles of ground to cover between the two horizons.
I suspect the real problem is your friend's attic installation -- especially if the siding on his home is stucco.
Doesn't matter he can work repeaters in your direction that are farther away -- if they're very high above ground, they could be LOS and then almost anything works. But that doesn't mean he'll hear you, if you're lower.
BTW "most" 2m repeater antennas are commercial 4-poles or Stationmasters that are 20' tall and have 5.5 to 6.0 dBd (not dBi) gain. That's a whale of a lot more gain than any kind of j-pole (or slim jim) design can possibly have. And usually they're up on tall towers or tall buildings...as such, there's really no comparison.
If he has stucco siding, an attic antenna will work poorly no matter what it is. Stucco is a terrible dielectric and is very lossy.
Slim Jims and J-poles cannot compete with large base antennas or beams. Trees in leaf can be a problem, right now I am having problems with them and APRS satellites, when there are not leaves on the trees signals are very good, but right now they suck. In comparison to 2m, 70cm works fine through trees. I had no problem getting a good signal form the ISS on 70cm but on two meters it sucks, so that could be part of the problem.
To cover 10 miles between line of site you need beams. If you have 70cm try that first.
Ok thanks for the additional info.
30 miles is a chip shot on simplex... I am using the generic 17' tall dual band vertical and often work stations 40-50 miles out when band conditions are flat. But I am feeding it with 5/8" hardline with .35db loss for the 60 foot run.
RG8X is horrible at 2 meters, you have almost 2db loss(3db is half your power!) on transmit and receive so that would be one spot to upgrade. LMR400 would bring losses to .6db which is a heck of a lot better and it is around $1 a foot so not out of line for price. Used hardline can be found on ebay for around the same price and bring that loss down even more but connectors are more expensive and can be a pain to assemble LDF 4-50b would bring losses down to .34db for example meaning more transmit power makes it to the antenna and more receive signal makes it to the radio!
On vhf there is no substitute for bigger antennas and better coax!
Is that the tram 1481?
I have been using a 1480 for years and it is a good antenna.