SWR'S jump with the amp

[998]

Ok heres what I got going on .....
I got a 10k antenna
My swr's is 1.1 with just the radio ....
1.1 with the amp on low ....
1.4 with the amp on high ....
Keep in mind this is a 2 pill I'm getting ready to install the 8 pill just want to get this straight first .
Think I should snip it a lil to get the swr lower now the swr's jumped that high with the 2 pill?????

[Dodgem250]

it's quite normal to have a higher SWR reading with an amp in line, this is why you should set the SWR with the amp in line, but, I'm quite sure someone will have to write the complete novel about how to do it, so, that's all I got for you.

[HotWheels]

Alot also depends on the length and design of your cabling. There is a formula using the velocity factor of the cables. Check this thread (http://www.mauldroppers.com/showthread.php?t=591) for a little more info into what I am talking about.

[zman350]

its gonna rise some with power on,but to get this even lower is trying to get your antenna just as low as it willl go on swrs.

[Fluxmaster]

I cant explain the technical reason but yea its very common for swr to go up a bit, those numbers are pretty normal.

[junkie 870]

i'm gonna tune my antennas for lowest reflected watts with my 4 pill in line...that's the plan ;)

[BumperJack]

junkie i would set the swr's first with the radio then do reflect with amp in line just my .02

[junkie 870]

yea, that's kinda what i meant to say lol...use one of them fancy mfj thingy deals lol

[MoJo]

Alot also depends on the length and design of your cabling. There is a formula using the velocity factor of the cables. Check this thread (http://http://www.mauldroppers.com/showthread.php?t=591) for a little more info into what I am talking about.

Here we go again...

Coax length doesn't mean shit when it comes to SWR tuning. You are tuning the antenna not the cable.

And the link that is supposed to help goes to a site that has a whole bunch of ads on it.

[junkie 870]

his link is now fixed...

[Stone_Wall_668]

I see the same thing with my setup.

SWR 1.1 with radio + amp in line but amp off.
kick the amp on and the swr goes up ~1.4......

Power on the amp low 50 - 350 swing modulating. no heat any where after talking for a LONG time. rev looks good to me its low and it will swing back w/ modulation?

If I crank things up to the max, rev goes WAY up but swings backward still while modulating. so will my FWD with everything maxed out......

Im thinking I need more amps to the amp, any ideas?


sw-668

[BOOTY MONSTER]

devide 492 by the cb center channel frequency (27.205) and then multiply that by the velocity factor of the coax being used . multiply that by 12 and you have the inch in length .

[Swamper]

Coax length can and will effect swr. If common mode currents are present then the cable can act as part of the antenna. If your antenna is a true 50 ohm load then coax length will not change swr one bit, but if you hook an antenna analyzer to your antenna I will guarantee you that it will not be 50 ohm's..

[Swamper]

I see the same thing with my setup.

SWR 1.1 with radio + amp in line but amp off.
kick the amp on and the swr goes up ~1.4......

Power on the amp low 50 - 350 swing modulating. no heat any where after talking for a LONG time. rev looks good to me its low and it will swing back w/ modulation?

If I crank things up to the max, rev goes WAY up but swings backward still while modulating. so will my FWD with everything maxed out......

Im thinking I need more amps to the amp, any ideas?


sw-668


If your swinging backwards your dk is too high..thats when a variable power comes in handy, it allows you to fine tune your dk..you cant do that with just a hi/lo switch..

[Stickman]

Ok I'm surprised that one of the antenna gurus hasn't chimed in on this 1. First Swamper is right you adjust the antenna not the coax. Lets start from the beginning here You adjust only the antenna and what I mean is nothing in line but the radio and meter than antenna. Jumper from radio should be as short as possible, I like to use a 3 inch piece myself, but I also use my bird for reflect rather than swr meter, this will work the same with most conventional meters, I have never used a jumper longer than a 1 foot piece but this is me. Now with nothing in line adjust for the lowest possible SWR or reflect. Once you have this adjustment done and you are happy, add amp in line, now this is where everyone gets screwed up, but put amp in line and leave meter back at radio, adjust for lowest SWR with your jumpers not the antenna, what we are doing here is matching the impedance of your system for the best possible performance. So what i mean by adjusting your coax is changing jumpers (3,6,9,12,etc) for the lowest readings you see on your meter, hard to do without a bird but can be done with a standard meter just not as accurate. Once you have the correct length from radio to amp with the lowest readings you can get, move the meter behind the first amp and repeat the process on the next amp. Once all is done go have fun, and also remember those conventional watt meters aren't that accurate for checking SWR with meters on, this is where a bird comes in to play, as most meters will show higher with amp on, there not designed to show swr with high watts in them. Just my 2 cents

[CamoRedneck]

Awesome advice Stickman

[Stone_Wall_668]

Thanks.....

stick,, now i see what ive been missing.....

swamper, ive got the l/h swx on the radio. think im going to chang it over to a pot now.......

sw-668

[Swamper]

its hard to trust most swr meters this is why I bought an antenna analyzer. My first meter was a Rat Shack and with no power it showed 1.1 swr, and then with the amps on it went totally crazy and pegged the meter. I have learned that for checking swr an analyzer is the most accurate and if your checking reflect a bird or the analyzer is your best bet..other than that anything else you cant put a whole lot of trust into as most meters will give you different readings.

[Stickman]

Yes Swamper you are right, these meters aren't designed for power to go threw them for the swr readings, If I remember right and i may still have it around here some where, but 1 of the manufacturer list in their manual not to ck swr with amps on.

[NVA Coax Cutter]

its hard to trust most swr meters this is why I bought an antenna analyzer. My first meter was a Rat Shack and with no power it showed 1.1 swr, and then with the amps on it went totally crazy and pegged the meter. I have learned that for checking swr an analyzer is the most accurate and if your checking reflect a bird or the analyzer is your best bet..other than that anything else you cant put a whole lot of trust into as most meters will give you different readings.

Refect is a representation of swr.What i do if i dont use a mfj.Tune antenna with barefoot radio and Rat shack meter.Get swr as low as posable.You know like low!!Then if you have a bird meter.Set Dead key with amp on.Install reflect slug Ckeck swr/reflect on 20,1,40.Ajust antenna up or down depending on your readings.Should be less then 2% of power going out.SWR should be less than 1.3.Sure it may work but not not as well as it should.May burn your amp up too.If over 1.3.

[BOOTY MONSTER]

stickman is . correct you tune the antenna , not the coax . for us folks using meters that cost $100 or less most of those can be fooled into false reading with some coax lengths . the way to avoid that is to use a half wavelength (or multiple of it for longer runs) of coax . im happy for folks that can afford $350 meters and $50 for each slug , but some of us arnt that fortunate . but fortunately for us there is a way to still get a true reading .

common mode currents are not caused by vswr . its another issue and a "ugly bauln" will take care of those .

[NVA Coax Cutter]

What is a half wave length????

[BOOTY MONSTER]

devide 492 by the cb center channel frequency (27.205) and then multiply that by the velocity factor of the coax being used . multiply that by 12 and you have the length in inches .


use multiples of that length for longer runs .

[Swamper]

If the load is a true 50 ohms such as a dummy load then coax length will have no effect on Swr. But since most antennas are not 50 ohms then changing coax length does effect Swr. Also common mode currents when they Are present, will cause the coax to act as part of the antenna. An easy test for common mode current is when your checking Swr grab your feedline and if the Swr meter reacts then you have current coming back down the braid of the coax. Just like when you touch your antenna with an Swr meter hooked to it the meter will react.

[Stone_Wall_668]

[QUOTE=Swamper;An easy test for common mode current is when your checking Swr grab your feedline and if the Swr meter reacts then you have current coming back down the braid of the coax.[\QUOTE]

Thats a nice easy way to tell if you have back feed? Then you can use the ..........

[QUOTE=Booty;
common mode currents are not caused by vswr . its another issue and a "ugly bauln" will take care of those .[\QUOTE]


There is a formula to get the windings and spacings, your making a matching xformer? to get the ant tuned to the system...........


sw-668

[BOOTY MONSTER]

examples of balun construction .
http://www.hamuniverse.com/balun.html

instructions for baluns @ 27MHz .
http://www.signalengineering.com/ultimate/coax_basics.html
"You can try to eliminate this situation (called "Common mode currents") by winding an "RF Choke". Wind about 6ft of RG-213 or RG-8 into a coil (6 to 8 turns). For RG-58 use 4ft with 6 to 8 turns. Wind the coax up, placing each turn right next to one another. Use electrical tape to secure turns together. You should place these as close to the antenna as possible. Right at the antenna coax connection point being optimum. Most times, you can verify that you have common mode currents flowing back down the coax by grabbing hold of the coax while transmitting and moving the coax around. You can watch the SWR waver by moving the coax while transmitting (don't speak into mic!). You have to do this with all the doors closed from inside the vehicle. SWR should waver, if you notice that SWR jumps rapidily between two values, you might have a intermitant (bad) connection in the connectors (PL-259s) on the coax. In most cases of "common mode currents", just grabbing the coax will cause the SWR to change."

[HotWheels]

Here we go again...

Coax length doesn't mean shit when it comes to SWR tuning. You are tuning the antenna not the cable.

And the link that is supposed to help goes to a site that has a whole bunch of ads on it.

Yet there is still nothing wrong with using the velocity factor when determining length of cabling but in further investigation, you are indeed somewhat correct. These are some issues that Booty Monster discussed in his last post. The link he gave has excellent information. No matter what is said, I think it all depends upon the individual and how each reaches the "perfect" 1:1 depending upon the actual equipment they have inline.

[BOOTY MONSTER]

the half wave length for coax is only to get a more accurate vswr reading from meters that are not high end equipment . it WILL NOT change the true vswr of the antenna , this is assuming that the coax is the proper type and in good working condition .

a bird meter is about 350 bucks with slugs about 50 bucks . i paid $80 for my 78ltw (sale at sparkies) and $200 for my 500 v used , i got my 2018 for $40 new and my 10k for $40 used . i refuse to buy a meter that cost more than my entire system . i got a PDC new for $70 . the best there is ? certianly not ! but good enough for me .

[Stickman]

You cheap ass Booty lol, But the links are very helpful to allow people to better understand coax and feedpoints.

[nn216]

If all components show 0 reflect your swr should show only a minor change power on or off. The problem with this statement is that in mobile operation Your transmitter and amplifier are so close to the antenna that they sometimes effect the accuracy of an in line meter . Meters that sample the signal without effecting the transmission line ,such as bird or para dynamics and some others are your best choice. The less reflected power the better your equipment will work.Reflect, and swr at the antenna cause heat and heat kills electronics. If you are not sure you have it right take it to ..your tech and have him check it out..good luck and have an asskicker of a day......

[BOOTY MONSTER]

im not so much cheap as i am poor . lol . but i try to find good things when i spend a few bucks . i got a killer deal on the 500v though !! i try to provide respectable links when i can so folks dont have to just take my opinion on things . plus this hobby seems to have very few absolutes , IMO so theres usualy many variables that the links cover so it saves me typing . im not only cheap but im lazy too ! i actually prefer to call it being energy efficent . :ihih:

[nn216]

If all components show 0 reflect your swr should show only a minor change power on or off. The problem with this statement is that in mobile operation Your transmitter and amplifier are so close to the antenna that they sometimes effect the accuracy of an in line meter . Meters that sample the signal without effecting the transmission line ,such as bird or para dynamics and some others are your best choice. The less reflected power the better your equipment will work.Reflect, and swr at the antenna cause heat and heat kills electronics. If you are not sure you have it right take it to ..your tech and have him check it out..good luck and have an asskicker of a day......

part 2 vol 1;reflect if you have 1watt reflect at 1oowatt dead key this can be expressed as a percentage of the total out put, in the case above the percentage would be 99%match. This means that 99% of applied power will be transmitted,regardless of the standing wave at the antenna.The antenna does not care what its swr is it will transmit all power delivered to it. If you have 10watts of reflected power you have a 90% match so in this case only 90% power would be transmitted.This is the break point, this is where you if you go much lower with your match you could damage you equipment. Trimming your coax to match impedance is a perfectly acceptable method to achieve full output of your equipment,as well as protect from overheating.Any loss in the coax will be minor do to the short run in a mobile.These facts are easy to verify with information contained in ARRL Antenna Book...Have Fun..P.S. please do reasearch for yourself before responding.