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W5SMD
08-28-2010, 04:57 PM
I looking at putting together a portable qrp setup, and I was wondering how long my feedline should be. I'm planning on using a pac12 vertical antenna with 15' radials.

Do I need to be outside that 15' circle with my rig, or would a 10' or 12' feedline be long enough?

Also, if I feed it with coax, would I need a balun, or is there another method that I could use to separate the feedline from the antenna?

NA0AA
08-28-2010, 05:34 PM
From a practical standpoint, I'd go with 25', just so if there was a convenient shade tree or something.

K8JD
08-28-2010, 06:50 PM
You may want to locate your operating position away from the near field of the antenna.
allow your self enough coax to reach the antenna's best location. Eg. away from any metal objects like buildings, camper trailers or vehicles. It will work best in the clear.

G0GQK
08-28-2010, 09:27 PM
He says its a portable QRP station, the RF from 5 watts is nothing ! Have your coax as long as you think you need, maybe 18 feet of RG58. and if you intend to sit outside andits sunny and there are no trees buy yourself a gazebo !

G0GQK

N9AAT
08-28-2010, 10:07 PM
They say that for every dB of cable loss you loose a dB of signal. That's true above 100 MHz, but down in the HF band where most of us live, it's kind of a moot point. Atmospheric noise levels are always so high that your cable will attenuate noise and signal equally ... to a limit. I'd say 100 ft is too much. If you get to the point where receive signals are say 3 s-units above noise at the rig then 1 s-unit after adding your cable, you've gone too far.

As far as transmit is concerned, yes the more you use the more you lose, but like JD says you don't want to be right next to your antenna. Surely 20 ft ought to do it. You can always calculate the loss by knowing the dB loss of your line then calculate the loss using dB = 20 log (P2/P1), or for your situation Rig Power X antilog (dB loss / 20) = antenna power.

KU3X
08-29-2010, 02:00 PM
When I set up portable, I take two different lengths of coax with me. My antennas are always wires in the air. The two different lengths are 35 feet and 65 feet of RG-8X. I first erect the antenna, if possible, so I can use the 35 foot length of coax. If I can get the antenna high enough, I may have to use the 65 foot length. If my station is located too far from the antenna, like under a nicer shade tree, I couple the two coaxs together and end up with a 100 foot length of coax.
Less coax means less losses, but I give myself an option. I've worked a lot of DX with only 5 watts and a 100 foot run of coax.
Rig: Ten Tec R2040 or IC-703+, both rigs at 5 watts max output.
Ant: OCF40 or dual band 20/40 meter dipole

Just a foot note: I've already erected a sloping 20 meter dipole and my rig was only 5 feet away from the antenna. The rig worked great and my body is not glowing from RF.

WS2L
08-29-2010, 02:05 PM
This is just my own preference but many may agree that RG-8X is a better feedline then RG-58.

K8ERV
08-29-2010, 08:38 PM
Old Abe got it right when he said a man's legs should be just long enuff to reach from his body to the ground.

I think there is a lesson in there somewhere.

TOM K8ERV Montrose Colo

KA5S
08-29-2010, 10:17 PM
I looking at putting together a portable qrp setup, and I was wondering how long my feedline should be. I'm planning on using a pac12 vertical antenna with 15' radials.

Do I need to be outside that 15' circle with my rig, or would a 10' or 12' feedline be long enough?

Also, if I feed it with coax, would I need a balun, or is there another method that I could use to separate the feedline from the antenna?

There are a lot of answers from your one innocent question!

Any antenna beats none... but a dipole almost always beats a vertical due to loss in the ground under the radials. Your feedline should always be long enough to reach from the rig/tuner to the antenna! Seriously, you may not have much choice where you sit, and where the antenna goes. I once hiked (http://xa.yimg.com/kq/groups/2460661/sn/623005239/name/n_a)into a site carrying two 24 foot croppy poles that I set up on a picnic table as a V (http://xa.yimg.com/kq/groups/2460661/tn/1727711764/name/n_a) with wire running up the arms and into the middle. Would have done better to run as a dipole center fed with twisted pair wire and ends taped down the arms. KQ6XA blew away more elaborate candidates at a shootout with one of these. Link with preview (http://preview.tinyurl.com/38pzodr) to Yahoo Groups photo.


Cortland
KA5S

KL7AJ
08-29-2010, 10:19 PM
I looking at putting together a portable qrp setup, and I was wondering how long my feedline should be. I'm planning on using a pac12 vertical antenna with 15' radials.

Do I need to be outside that 15' circle with my rig, or would a 10' or 12' feedline be long enough?

Also, if I feed it with coax, would I need a balun, or is there another method that I could use to separate the feedline from the antenna?

A good rule of thumb is that the transmission line should be no shorter than the distance between the transmitter to the antenna.


Eric

KC8VWM
08-29-2010, 11:16 PM
How long is a piece of rope?

A balun is useful for transforming the impedance between the antenna and the feedline however if the intention is to keep RF off the feedline, you will probably want to use a choke.

I usually avoid using baluns when operating QRP because I generally want to minimize any and all possible insertion losses occurring between the transmitter and the antenna. Sometimes, I don't even use PL259 or SO 239 connectors and I just make direct connections !

Sometimes, I don't even use any feedline at all for that matter. I just plug the antenna directly into the transmitter and no feedline losses occur because there is no feedline !!

K8ERV
08-29-2010, 11:17 PM
A good rule of thumb is that the transmission line should be no shorter than the distance between the transmitter to the antenna.


Eric

I said that.

TOM K8ERV Montrose Colo

W5SMD
08-29-2010, 11:21 PM
A good rule of thumb is that the transmission line should be no shorter than the distance between the transmitter to the antenna.


Eric

Ok, I'll rephrase my question. What is the minimum distance that I could set up my rig from the antenna and still have everything work alright? From the replies I've gotten, it seems that 18' to 25' is the area that I should be looking for.

As far as a choke, would I need to buy one, or are they difficult to make? I have to pass all of this past the CFO (my wife) since I'm in school and she is currently bringing home the money.

W0IS
08-30-2010, 03:31 AM
Yes, 18-25 feet sounds good.

As far as everything working OK, there's really not going to be any practical difference if you're sitting right next to the antenna or if you are away from it. Yes, the presence of a six foot long conductive object (you) might make some theoretical difference. But as a practical matter, it won't (unless you're actually touching the antenna or something).

The piece of coax that I use for exactly the same kind of operation is about 10 feet long. A longer piece would sometimes be helpful, but only for my convenience (sitting in the shade, etc.)

You can crunch the numbers using some online RF exposure worksheet, but 5 watts of RF a few feet away from you should be well below the limits.

Incidentally, I just put up a dipole for the band(s) I intend to operate. On those rare occasions when I can't find the required supports, then I run a quarter-wave piece of wire as vertical as possible, and then have another wire(s) along the ground.

Occasionally, I dispense with the coax completely. I just plug a quarter-wave piece of wire into the back of the rig, and then run the radials to the radio's chasis.

At one time, I had some pre-measured wires for various bands. I had an alligator clip on one end, for the times I used it as a dipole, and a banana plug on the other end, for the occasions when I plugged it right into the rig. Being that organized is very out of character for me, so those pieces of wire got lost a long time ago, and now I just cut the wire as needed and just twist it on to wherever I need to connect it.

WA4OTD
08-30-2010, 04:02 AM
Have to frame this answer! Good one!

On vacation-spring break I operated 16' vertical with 2 above ground ground wires and I was only 8-10 feet from the vertical and I operated 40M-10M for a week.


A good rule of thumb is that the transmission line should be no shorter than the distance between the transmitter to the antenna.


Eric

KC8VWM
08-30-2010, 04:41 AM
Occasionally, I dispense with the coax completely. I just plug a quarter-wave piece of wire into the back of the rig, and then run the radials to the radio's chasis.

At one time, I had some pre-measured wires for various bands. I had an alligator clip on one end, for the times I used it as a dipole, and a banana plug on the other end, for the occasions when I plugged it right into the rig. Being that organized is very out of character for me, so those pieces of wire got lost a long time ago, and now I just cut the wire as needed and just twist it on to wherever I need to connect it.

Now this guy is a true QRP operator. :p

I have to admit I am not that organized either, so one solution I came up with in the past was to use two single bare copper wires cut for the 30 meter band. It was painted on various spots indicating quater wave resonance for various bands from 30 - 6 meters.

All you needed to do was pull the antenna wire out of storage (I stuffed the two "antenna wires" inside two film cans)

Once removed, find the paint mark on the wire for the desired band you wanted to operate on and simply fold the rest of the bare copper wire over itself.

Then it would be resonant on the desired band without the need of carrying extra antenna wires cut for various other bands.

Incidently I found out that I could also use the film cans I used to store the wire antenna as antenna insulators too. I used a roll of kite string attached to suspend my wire antenna using the film cans as antenna insulators. So the film cans served a dual purpose and this entire portable QRP antenna arrangement could fit inside two film cans in my shirt pocket. I could connect the two wire antenna's together to operate on other bands as a single vertical, as an end fed zepp configuration. or I could operate the wire as a "V" dipole when connected directly to the back of the rig, (kind of like extra long rabbit ears on a TV set sort of thing - The opposite to an "inverted" V) I usually ditched the feedline unless I was operating from inside a structure like a building, an RV or a tent. Worked great and it was dirt cheap too. I suppose you could sell something like that on the commercial QRP antenna market for $200.00 ahaha. :)

W1GUH
08-30-2010, 01:35 PM
Ok, I'll rephrase my question. What is the minimum distance that I could set up my rig from the antenna and still have everything work alright? From the replies I've gotten, it seems that 18' to 25' is the area that I should be looking for.

As far as a choke, would I need to buy one, or are they difficult to make? I have to pass all of this past the CFO (my wife) since I'm in school and she is currently bringing home the money.

Or, you can run balanced line and a true balanced tuner & not worry about baluns, chokes, and the like.

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