What length and type of coax are you all using for your Go-Boxes? Starting my project this weekend. I have the dry box, 857D, VX-170, tuner, Signalink, few batteries and battery tender.

30-40 feet in length.

2 meters, 20,40,80.

LMR240 ultraflex.

Perfect thanks.

To keep it simple. 50ft minimum length for freq down to 40m. 100ft minimum for 80m. 50 feet of coax will act as a quarter wave stub on 80m. AFM clued me in on this and verified it myself.

RG-8X works fine for the HF bands but losses becomes an issue on VHF. I use LMR-240 Ultraflex at a minimum for VHF. Prefer LMR-400 on VHF/UHF but I'm not carting that bulky heavy stuff around for a field situation.

I did not realize that I needed a minimum of 100 feet of coax for my 80 meter NVIS dipole. I am trying to keep one box self contained, and dont think I will be able to fit 100 feet in. I will test this. Thanks.

Use the RG-58 type that I suggested.

how so?

consider two cases –– the first where the antenna itself is a reasonable match to the coax's 50ohm characteristic impedance, and a second where it is not.

ar-jedi

A 1/4 wave of coax is NOT a "shorted stub", it is however a transformer. It will take a high impedence and flip it to low and vice versa. Under most conditions this isn't going to make an antenna mismatch any worse, just different.

Other than avoiding a 1/4 wave or other odd 1/4 wave resonant length there is no need to pick any particular length of cable. If you have a reasonably matched load there is no need to even worry about the 1/4 wave resonances.

so, this particular "perfectly good" antenna presents a "reasonable" load to a transmitter directly connected to it. in other words, it is roughly 50ohms resistive. is this correct?


how? you have already asserted that the antenna presents a "reasonable" load which approximates a 50ohm impedance, and this benign load is now terminating the length of coax. so we have a 50ohm source, a 50ohm impedance transmission line, and a (roughly) 50ohm load. how did we end up with a high SWR? please explain this further.


you are now measuring the impedance of a different system: a 50ohm characteristic impedance coax terminated with a very high (infinite) impedance. this configuration results in a marked reflection at the far end, and an attendant high SWR. or said in RF speak, the return loss (S11 parameter) is low. how is the impedance of the unterminated coax ("6 ohms") the same as the impedance ("6 ohms") of the combination of the coax and the antenna –– the latter which you have stated approximates a 50ohm load?


an unterminated halfwave length of coax has a very feedpoint high impedance, because of the reflection at the far end. a halfwave length of coax terminated with a 50ohm load (or nearly so) at the far end has a 50ohm system impedance. in this case there is no (or little) reflection, and the SWR is low (return loss = S11 = high).

we can further hypothesize and extrapolate from these two examples that any length of coax, if terminated with an impedance close the to coax's characteristic impedance, has no impact on the overall antenna system impedance presented to the transmitter. said another way, adding/subtracting coax will result in more/less resistive loss, but the SWR will remain low despite the varied length of the coax. is this not the case?



an unterminated 1/4 wavelength long piece of coax results in a very low impedance ("shorted") stub –– because there is a high (infinite) impedance present at the end of it. put a 50ohm termination on the end, such as a well-behaved antenna, and you have a (nearly) 1:1 SWR and no "short".

ar-jedi

Coax length is also being discussed on the Buddipole yahoo group. This link was posted.

https://docs. google.com/ leaf?id=0B7IoW_ mP1hrhMTdmZDRhMj EtOTcyMy00MDlmLW IyYzYtODk0NjA2YT U3ZmM2&sort= name&layout= list&num= 50




From the ARRL Antenna Handbook.

Just like that... here's mine.