antenna length

[hambo1981]

Hello all. I am a newbie to radio and want to know if an 17.3ft dipole will receive 21mhz 15m band. I am using a 10 and 12m radio for listening but have not heard any tx on 15m. The radio is a ranger 2950dx that covers the 10 and 12m bands but it has also been modded to receive 15m. I understand that without realignment etc tx on 21mhz will never work but was told it should be fine on rx. Any suggestions?

[Barnbilly]

you may hear some signals on it but it wont be very good on 21mhz because of the impedance mismatch presented to the receiver
The receiver is designed to work with 50 ohm impedance a 17 foot dipole will not present 50 ohms ideally you want about 22 foot plus or minus
a bit although the nhf bands have been very poor lately not sure if 15 is seeing much activity atm
also I am not too sure how well the receiver will work that far away from its design frequency

[Buick Mackane]

If its for receiving purposes only then it doesnt need to be spot on, But best if it is, click on the link below, It will give you the exact lengths needed to build a dipole (easily done) Just enter the frequency your interested in (centre of band) is best, And hit calculate

http://www.csgnetwork.com/antennaedcalc.html

[hambo1981]

Thanks for the prompt replies. I have set up a cb frequency dipole coupled with a..... now bear with me as I'm a newbie..... an rf choke/ ugly balun as advised by a club. This was apparently needed to match unbalanced coax to a balanced antenna. The dipole was a shop purchased sirio with gamma match. I know little of these things.... yet.
However when I purchased the equipment from a well known supplier they said a balun/ rf choke that I fitted was not required and they had never heard such nonsense so I am quite confused.
From how I understand it the rf choke, which by the way was a length of coax wrapped around a length of piping was to not only present a correct feed to the antenna but to reduce rf feedback along the screen of the coax over a larger frequency range than would normally be a usable range for an antenna tuned for 27mhz.
ie the swr would be lower at say 25.5 with the choke than without.
Like most of you when you started off I have a lot of questions that I am getting very different answers to. I want to set every thing up as well as I can and I appreciate your responses.

[Tigersaw]

Are you just receiving or transmitting as well, its not clear.
The choke is not required for receiving, and the ugly balun variety is only (marginally) effective for the range of frequencies its wound for.

[hambo1981]

Hi. Transmitting only on cept at the minute until I do my foundation exam in 6 weeks. The ugly balun I was told was essential when connecting a coax feed to a dipole. Also I was told that it would help to give an acceptable swr on frequencies slightly outside what the antenna is tuned for.
For example I would have good swr at 27mhz but poor at 28 upwards without the coax and pipe winding. With it it would give a better swr. Is this not the case? Also does that antenna require it for the unbalanced coax to balanced antenna connection? Thank you again for any advice.

[Tigersaw]

The height above ground of your dipole, the length of the elements, the position of your choke along the co-ax feed will all make a difference that can be beneficial or detrimental - what works for one doesn't for another which often accounts for the huge amount of advise out there.
Its likely that your dipole (any dipole) is not exactly at the right dimensions or height or material to present a resistive 50 ohm match to your co-ax, and so that also becomes part of the radiating antenna. The idea of the choke is to stop that radiation being returned to your shack.
Dont get hung up on believing a low swr means a good antenna. A dummy load will give you the best ever swr but wont radiate well.
Experiment, move things about, measure swr in different places not just at the radio, try your dipole as a V, get radio reports as you go and you will begin to find what works best for you and why. Thats all part of the fun.

[Barnbilly]

And just to be clear coax wrapped around a pipe doesnt make a balun, it makes a choke, 2 different things
as the above post experimentation s the key,play about with the antenna and find what works, really good way of learning

Half of what you get told by "Bob" down the pub is anecdotal and as any scientist will tell you anecdotes are not data

[Admiral]

I've never used a BalUn on a TX feed, I thought that's what ATUs were for???