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392
| | Posted on Saturday, October 11, 2003 - 9:10 am: | 
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OK I've got a yaesu ft817 in the vehicle, low swr plus 35 w booster, larson dual band antenna. Can hit repeaters 30 plus miles away.
At home I've got an Alinco dr605, disccone antenna and flat swr.
At 460 mhz I get full power on both units. Why can't I talk from base to moble any further than 1 mile. The disc cone is 35 feet in the air and also hits repeaters 20 - 30 miles away. SWR checks out, power checks out, no ctcss are activated. We're on GMRS freqs. It seems like this setup should get several miles but it only gets about one. I'm trying to work this setup in with my business portables.
Any thoughts??
thanks,
392 |
Bruce
| | Posted on Saturday, October 11, 2003 - 1:43 pm: |
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Hummmmmm
Ok at 460 a discone is fine for general work but i have found BIG improvements using a 7-11 db gain colinear. The larson ( and other ) dual band whips (the through the glass are the worse) realy are a copermise at best again they do just fine to repeaters and there is a 2nd problem i see here and thats bandwith. Using a 440 ant 0n 460 is again going to lower the gain we found with MURS 2 meter antennas worked ..... tuned for MURS they worked a lot better. I would if i wanted to do this right get a 7-11 db gain base ant there are clones of the dimond out there on ebay for 70-100 bucks i got the big one 17 foot tall 6- 1/2 waves stacked on 440 and it does work on 460 gmrs better than a disone MUCH better and had the little one $70 4- 1/2 waves stacked in a 10 foot tube ... again you COULD tell it from a discone.
you can build a gain J-POLE 6 db of gain from copper pipe and ill bet it will walk circles around a discone. I have several discones on 2 and 440 and receiving around them they are fine but you need GAIN and a discone just doesn't have any.Build a J POLE or Check e-bay for .....
9/11 dB Gain 146/440 MHz. Base Antenna ....it an't cheep but mine works fine.
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392
| | Posted on Saturday, October 11, 2003 - 5:29 pm: |
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thanks Bruce, I'm going to try again and hook the dr605 to my cushcrafe 3 elem beam. It's tuned for 460 and SWR's are very low. It just didn't seem right that my wife couldn't hear me only a few blocks away on my mobile and that antenna was also cut to match 460, swr almost flat and 30 watts, but with the dame unit I can hit repeaters 20 miles away just fine. Something seems wrong on both ends?
Thanks,
Tim |
ss8541
| | Posted on Saturday, October 11, 2003 - 6:18 pm: |
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here is one thought. what type of coax are you using? if you are using rg58 for mobile and base, then this is your answer. the loss for a standard 17ft run of rg58 with nmo type antennas is about 30%. so if your radio is putting out 100w then only 70w is actually making it to the antenna. it would be even a greater loss for the base.
this type of coax will also give you a much better swr than you actually have because of the loss. so while you are showing a flat swr, this may not be the case. it could actually be a bad swr that looks good, or even great, at the opposite/radio end of the coax. a swr combined with high coax loss can make the actual radiated power even less than just the loss in the coax with a well tuned antenna. so you may actually be losing 50% of your power or more.
this could explain why you have a flat swr but yet can't talk a mile away base to mobile. as for why you can hit repeaters 30miles away with both, i can do this with my 4w uhf walkie(depending on height of rptr), so hitting a rptr 30 miles away isn't saying much. not being smart, just stating the facts. |
Znut
| | Posted on Saturday, October 11, 2003 - 6:52 pm: |
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A discone has no gain and a wierd pattern. Try a different antenna for base to mobile work. I would try a collinear, it keeps the signal where you need it. A j-pole, again would be fine for a repeater but the radiation patterns are unpredictable based mostly on how they are fed and decoupled but usually have a higher angle.
Simply put, most antennas other than a vertical dipole, a beam, or a collinear(stacked) will have a pattern that radiates at an angle more or less above the horizon. Your signal will miss anything that is not WAY in the air.
Another factor is your terrain. If you have hills and or buildings between you and the mobile AT ALL you will not have much range on UHF. A little foliage is not too bad but solid objects will block your signal. |
Bigbob
| | Posted on Saturday, October 11, 2003 - 7:21 pm: |
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Yeah if you could carry a 150 foot tower on your mobile then you shouldn't have any problems hitting that either. |
Bruce
| | Posted on Saturday, October 11, 2003 - 8:10 pm: |
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coax was next at 460 you need good coax i measured 13 db per 100 foot at 460 in a random peice of rg-58 and 10 db im mini 8. Even so the BIG change was the antenna a discone has NO gain if you go to a 11 db gain antenna its like 13 TIMES the power and as you can see thats a bunch!
On 440 i use a good rg-8 since my run is only 35 foot but good coax also pays for its self cheep stuff is a plan for failure |
ss8541
| | Posted on Saturday, October 11, 2003 - 8:57 pm: |
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to put it simply, if you had a uhf rubber ducky like the one on a ht, on your car and one on the base, you should be able to talk more than a mile if both antennas are resonant and the proper coax is used.
discone antenna or not, you should be able to talk more than a mile base to mobile if everything is installed correctly and the right type of coax is used.
think about it, even the gmrs radio ht's can talk more than a mile to each other in open terrain. this fact alone shows that you have got something seriously wrong at one location, and i would guess at the base. i say this because it has the longest coax run, which in turn could/would show everthing as good on a swr/wattmeter(reflection wise) when it really isn't. at 35ft up even a 1/2 watt of radiated power could be enough to talk into a repeater 30miles away. put it wouldn't be enough to get to your mobile down the road.
yes, solid objects block uhf freqs, but they also reflect uhf freqs well also. uhf will bounce around and find a way around an object. we have a large campbell soup factory here loaded with metal cans and machinery. the 300 radios there bounce around this stuff all day and make it to the rptrs.
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bigbob, i don't follow your post at all. |
392
| | Posted on Saturday, October 11, 2003 - 9:50 pm: |
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I didn't think about the coax. I am running the thin stuff on the base and I think it's at least 50 feet, perhaps more. I have some rg8u, 100 feet and a cushcraft 3 element beam. I will try this combination as soon as the rain stops. Looks like the disccone will have to go for now.
thanks everyone again.
tim/392 |
392
| | Posted on Saturday, October 11, 2003 - 10:18 pm: |
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OK I need to understand something. I was always told that in the city 400 mhz range was the best and easily passes through structures. In my work we use everything from 150 mhz to 900 trunked and I can't tell what works best. So if this not true what is the best freq range for local talk in the city? |
ss8541
| | Posted on Saturday, October 11, 2003 - 10:47 pm: |
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392, i had a very good feeling that this is what you would find. i work with uhf freqs all day for a living and hobby.
now lets take a worst case scenario here and say that you are running 75ft of rg58. the loss here is 7.5db or a loss of 83% of your power. so if your txer is putting in 30w, only 5w would make it to the antenna. now if that antenna had a 3:1swr, there would 25% of this not being radiated, knocking you down to 3.25w of radiated power. this relates to a power reduction of almost 10db of total attenuation.
on top of this the reflected power would also be reduced by 83% at the radios end, showing a excellent swr. example; out of the 5w making it to the antenna, 1.75w is being reflected(25% of 5w). out of this 1.75w being reflected, 83% is being burnt up by the coax. so only .3w is being measured as the reflected power on the radios end. when all the math is done, this gives an swr of less than 1.1:1 when there is actually a 3:1 swr. |
Bruce
| | Posted on Saturday, October 11, 2003 - 10:55 pm: |
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CORRECT
800 is better than 400 it penitrates windows and cracks better HOWEVER total loss is much higher on 800 and range is shorter .... if you use 400 and 2 meters you will see in a car 440 will work better however the range on 2 meters will be better over all if both systems are the same. MURS if you do it right has a range of between 5-15 miles base to moble while the best ive done on GMRS is about 5 miles.
Here at the jail we use 400 meg the county 800 system dont have a prayer
in this place several feet of concreat and steel bars when you add them up and our 150 watt putput repeater into a 12 db gain ant is almost lost it gets down below .5UV and we have remote recievers so that radios can get out from thoes spots. 800 in here would have problems going between rooms unless it found a window and everything in a jail is reinforced. Radio is so much fun ..... |
Znut
| | Posted on Sunday, October 12, 2003 - 1:04 pm: |
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ss8541
True, UHF is excellent for warehouse conditions. The signal is reflected all over due to the metal structure. Outside, however, a hill or large building will reflect the signal, sure, mostly the wrong way if the object is between the two stations. I think Bruce and I should also have mentioned the coax.
Bruce also touched on path loss. As freq increases, path loss increases. Depending on conditions, you will see around a "many" (not doing the math for exact #'s) times difference between 27mhz and 450mhz. Line loss the same way.
Still, with an antenna being 35' your range should be more than a mile. With 30 watts, good antenna and coax you will be able to wk a few miles.
The trunking system works similar to cell phone systems. The coverage is due to the number of repeaters or nodes not the range of the radios.
150mhz radios have a MUCH greater range especially with one base or repeater in a central location. |
Bigbob
| | Posted on Sunday, October 12, 2003 - 3:13 pm: |
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ss8541,line of sight transmissions depend on towers for great distances like 30 miles,was just being facetious,didn't mean to add confusion. |
392
| | Posted on Sunday, October 12, 2003 - 6:10 pm: |
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I tried the same setup on my cushcraft 3 elem and distance was less than the discone. About half a mile at best in residential area. This is strange that I can talk to other hams, simplex 50 miles away but can't hit my own vehicle a few blocks away with this much power. The handhelds work just as well with 2 watts. It's got to be a total antenna mis-match on both ends, even though everything checks out on the swr meter.
Oh well, just trying to have some fun on GMRS. I'll stay with it until I figure it out.
392 |
392
| | Posted on Sunday, October 12, 2003 - 10:58 pm: |
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Bruce was right, MURS Rocks! I did a side by side comparrison. My beam is dual band as is my alinco. On the GMRS I got a few blocks. Switched over to a MURS channel and no comparrison. I drove 10 miles away and still pegged the meter on both ends. It even out performed 26.535 which always works well. My abs1600 is 50 feet, the cushcraft 3 elem is 30 feet but 151mhz rocks, crystal clear. GMRS won't cut it around town, I'll play on 151 for a while!! Move tht antenna to the 50 foot pole. Isn't this fun!!
392 |
ss8541
| | Posted on Monday, October 13, 2003 - 11:42 pm: |
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i used the factory as an example of how uhf signals in close range would find ways around solid objects. i used this in relation to his base/mobile closeness. i was trying to be nice in saying that objects being in the way and/or path loss was definitely not his problem as to why he couldn't talk a mile. at a mile there would have to be enough solid objects to bounce the signal around before enough loss came about to kill his signal. due to the fact that he said both systems 'showed' a flat swr, meant one thing; coax with too much loss making a bad swr look good(and compounding the loss problem). if he really did have a good swr, and the right type of coax, he would have been talking more than a mile.
there is talk of LOS(line of sight) but you all must remember that there is 4 parts to line of sight. the direct ray, reflected ray, the refracted ray, and the diffracted ray. these will all either reinforce each other or cancel each other. with a mobile in motion, you are going to have both (a.k.a., picket fencing) not total cancellation like he would have thought to have by some of the statements here.
also, he said he lived in a town, so i ASSumed that there were not tall buildings crowded around him(which would be a city). i still assume this since he was able to talk 10 miles with MURS freqs. if the large buildings in a city were keeping him from talking with uhf more than a mile, he definitely wouldn't be able to use vhf freqs at 10 miles better than he could hf(26mhz) freqs(like he said above). this fact also throws the LOS goes against the LOS argument.
this is a little off subject, but since it was brought up, the coverage of trunking systems depend on the type of trunking system and the # of sites. a single site trunking system has the same coverage as a single rptr(especially if it is using a tx combiner with 1 antenna and an rx multicoupler with 1 antenna). so here the range does depend on the radios. this is mainly true for ltr overaly trunking/smartrunk in the 450 range. for 800mhz, yes there is usually more than one site, hence a broader system 'footprint'.
as to 150 having a much greater range, this is untrue to some extent. the higher gain antennas at these freqs make up for much of the loss. under the same conditions(same power, same coax loss, antenna gain, etc) yes, vhf does have a greater range, but i wouldn't say 'much' greater. if that was the case police dpts, etc. would have all went back to low band vhf and high band vhf years ago. once again, higher gain antennas for mobile/base/rptr make up for much of this loss.
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392, don't rule out a problem in the mobile or base or both because you could communicate 10 miles with vhf and only a few blocks with uhf. this proves even more that something is not right with one or both antenna systems in the uhf band.
here's also something else that many do not realize. if you have a lot of noise on a cb freq, it actually desenses the rxer. you wouldn't have had this problem with the MURS freqs, and may be a reason why you could talk further than with the cb. this is why i tell all my customers to use the rf gain 1st and then the squelch. you will then be able to here things that you wouldn't if you just used the squelch(too much desense with a high level of noise). just thought i would throw this in for good measure. may not be the case here since your below ch1, and the noise most likely isn't as high as it would be on the 'legal' channels. |
392
| | Posted on Tuesday, October 14, 2003 - 12:56 am: |
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It seems that coax is a major player here so for uhf/vhf what is the best coax to buy? |
707
| | Posted on Tuesday, October 14, 2003 - 1:53 am: |
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Bad enough to lose all that power output, but even worse yet, you are losing all your receive!
Go with some nice heliax or at least LMR-400 |
ss8541
| | Posted on Tuesday, October 14, 2003 - 10:14 pm: |
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707 is right, for the money you can't beat lmr-400. you also want only enough to connect the radio to the antenna. the longer the length the greater the loss. |
392
| | Posted on Tuesday, October 14, 2003 - 10:45 pm: |
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Thanks guys, I will order a 60 foot roll of this stuff tonight. Hope Copper's has it!
392 |
ss8541
| | Posted on Tuesday, October 14, 2003 - 11:17 pm: |
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oh yeah, one other thing...use the shortest length of coax possible to check/set swr. the longer the length, the better the 'displayed' swr will be. so don't set the swr with the length that you are going to run permanently since it will give you a better swr than you actually have(problem you have now). |
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