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.

For what frequency and what coax length?

30-40 feet in length.  

2 meters, 20,40,80.

Good quality RG-8X is really handy for HF. In shorter lengths works OK for VHF

LMR240 ultraflex.

Times-Microwave LMR240ultraflex is pretty handy stuff, very flexible and loss should be very reasonable at 2m (~1.75dB @ 146MHz for 50 feet).  Belden 9258 is similar.

A good quality flexible RG58 size foam cable like Belden 8219 would be perfectly acceptable for the HF frequencies, for that matter could get by with that for 2m for a run that short with about 1dB additional loss on 2m for a 50 foot run.

Perfect thanks.

I would recommend longer than 30-40 feet especially for the HF antenna.   I'd probably have 100' of Belden 8219 for HF (1.3dB loss at 10Mhz for 100').

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 use Mini-8 (RG-8X) for my HF go kit.  I have it on round extension cord reels.  And as stated, works fine in shorter lengths for VHF.

That is EXACTLY what I have, same reel and all.  

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.

The formula is: (468 / Freq) * Vf = Minimum coax length.
Freq is the lowest frequency of the band your working with. 3.500 Mhz for the 80m band. Vf is velocity factor of the coax you're working with. You can measure the velocity factor with an antenna analyser such as the MFJ-269 or find a reasonable value by searching online. For RG-8X I find .85 give or take a little. Run the numbers and you come up with 113.7 ft as the half wave. 100 ft is close enough.

You can test this easily with an antenna analyser. Run out a 50 ft length of coax. No terminination except the connector. No dummy load, no antenna. Run the analyser through the band you're targeting making note of where the coax starts to resonate. Sure enough a 50ft length of coax will start to resonate toward the upper end of the 80m band. Your RF radiating out of the coax is not a good thing. Add another 50 ft of coax and try again. Doesn't resonate and problem solved.

Use the RG-58 type that I suggested.

468 / 3.900 mhz x .84 (velocity factor of RG-8X)  =  100.8'  for a halfwave at 3.900 mhz.

It's also appx 2 halfwaves at 40 meters, or 4 halfwaves at 20 meters.

Coax length DOES matter.

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 perfectly good antenna at 1.4 or 1.5:1 SWR on, say, 80 meters.

Put 50' of RG-8X to it, and the readings are squirrely, high SWR, 6 ohms... as if shorted out.

Unscrew the coax from the antenna, check with analyzer again, 6 ohms.  What?

It is a 1/4 wave... and is a shorted stub.  

SWR readings are good on the antenna on 40 and 20 meters, as it should be, but bad on 80 meters.

Put 100' of RG-8X, which is a halfwave at 80 meters, and everything is good.

A halfwave of feedline repeats the antenna's impedance every half wavelength and multiples thereof.

A quarter wavelength (50' at 3.8-3.9 mhz) is a shorted stub.  75', 80', 37.573', are all OK at 80 meters.  It's just that pesky 1/4 wavelength that is the problem.

So, some guys say BS, feedline length does not matter, and others say it does.  My actual measurements say it does.  

Here it is in writing:

The Fifty Ohm Enigma, or, “Have you got a match?”

http://emrcorp.com/techinfo/PDF/The_Fifty_Ohm_Enigma_w_Logo_03-01-10.pdf    < That is a pdf download.

Particularly page 10 - Coaxial Line Segments As “Repeater” Sections and “Stubs.”

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.

Or a guy could be a dumbass like me and just use an SG-230 at the antenna feed point....making feedline length pretty much a moot point.

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

Well, I get a reading of 1.4:1 SWR with 100' coax at 3.920 mhz.


Here's another.

http://www.qsl.net/co8tw/coax.htm



What I do know is, 100' of RG-8X works fine on 75 m.  50' doesn't.

Until this experience I though coax length didn't matter.

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.

Son of a bitch...  Pure genius.



 

Just like that... here's mine.