g7fek balun position - Page 1
Forum section: electronics / rf-microwave

author: p.larner
author_rank: Frequent Contributor
timestamp: October 25, 2023, 08:16:00 pm
content: assuming i need a balun for this antenna,is it best at the antenna or at the rig end of the coax,also what is best?,i have a 1-1 a 4-1 and a 9-1 type.

author: A.Z.
author_rank: Super Contributor
timestamp: October 25, 2023, 08:56:19 pm
content: Quote from: p.larner on October 25, 2023, 08:16:00 pmfrom your questions, it sounds like you don't know how a balun does and how it works, and then, apparently you also don't seem to understand how the G7FEK antenna works and how/why it was designed the way it is

author: p.larner
author_rank: Frequent Contributor
timestamp: October 26, 2023, 06:27:56 am
content: i was thinkng about sticking the balun further away from the antenna so the coax shield was a part counterpoise/ground/radial.

author: Andy Chee
author_rank: Super Contributor
timestamp: October 27, 2023, 05:16:21 am
content: Quote from: p.larner on October 26, 2023, 06:27:56 amFor this purpose, you need to use a choking balun to stop RF on the shield, by placing the choke at your desired length of coax shield counterpoise.Not all baluns are choking baluns. For example, the Guanella 1:1 is a choking balun, but the Guanella 4:1 is not.If you require impedance transformation, then you should use two baluns; the first balun on the antenna side does the impedance transformation, the second balun on the radio side does the choking.A casual look at the G7FEK antenna suggests no impedance transformation is necessary.

author: A.Z.
author_rank: Super Contributor
timestamp: October 27, 2023, 04:54:37 pm
content: Quote from: Andy Chee on October 27, 2023, 05:16:21 amTo be precise, the Guanella 4:1 is still a choke, but it offers about half choking impedance than the 1:1

author: Andy Chee
author_rank: Super Contributor
timestamp: October 28, 2023, 04:32:35 am
content: Quote from: A.Z. on October 27, 2023, 04:54:37 pmYou're right. I think I was meant to say Ruthroff 4:1 as I typed that post, but Guanella came out.As an aside, there are designs floating out there on the interweb of a "single-core" Guanella 4:1. This is NOT a Guanella, this is effectively a Ruthroff.

author: A.Z.
author_rank: Super Contributor
timestamp: October 28, 2023, 07:25:14 am
content: Quote from: Andy Chee on October 28, 2023, 04:32:35 amwell, no those are still Guanella, but built the wrong wayhttps://www.dj0ip.de/balun-stuff/1-vs-2-core-baluns/the Ruthroff is a "voltage" BalUn, and it's wound in a different way, yet a Ruthroff is still useful, for example one may use a 4:1 Ruthroff to perform impedance transformation and follow it with a Guanella 1:1 to balance currents and achieve chocking, the result in my experience is pretty good

author: A.Z.
author_rank: Super Contributor
timestamp: October 28, 2023, 07:58:07 am
content: anyhow. getting back to the G7FEK antennahttps://www.g7fek.co.uk/blogus/newsshow.php?page=G7FEK_Limited_space_anten_29507the PDF found at the above page suggest putting a choke (1:1 current BalUn) at the point where the coax enters the building, that means using the coax as part of the counterpoise system and, in some way, as an impedance adapter, I don't like it, but that's what the author says

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: October 28, 2023, 08:37:05 am
content: Putting a common mode choke at the antenna feed point still allows RF pickup on the coax outer, putting one at the shack end eliminates the RF on the coax entering the shack.If the coax runs parallel to the horizontal antenna wire, even if the coax is laid on the ground, it is liable to carry some RF on the outer. Whether that RF causes problems, or results in noise pickup, is hard to guess.You can measure common mode current easily, although very few seem to bother. GM3SEK has posted an old RadCom circuit showing just how simple it is to make and calibrate such a meter:http://www.ifwtech.co.uk/g3sek/clamp-on/clamp-on.htmIncidentally, wind your coax through an FT240-31 toroid, it makes a much better common mode choke than an air cored coil. Typical choke winding details attached. Use PTFE coax for high power.SJ

author: A.Z.
author_rank: Super Contributor
timestamp: October 28, 2023, 02:09:20 pm
content: it depends from how the coax is routed, and then, #31 is fine from 160 to 40 meters, but going up in frequency, #43 is better, so ideally one may want to use two or even three chokes realized using different core materials.

author: A.Z.
author_rank: Super Contributor
timestamp: October 28, 2023, 07:32:27 pm
content: Quote from: A.Z. on October 28, 2023, 07:58:07 amalso seehttps://q82.uk/g7fek

author: Andy Chee
author_rank: Super Contributor
timestamp: October 30, 2023, 06:56:01 am
content: Quote from: A.Z. on October 28, 2023, 07:25:14 amIf it's built the wrong way, can one really call it Guanella?QuoteYep, I use this Ruthroff 4:1 & Guanella 1:1 combination on my own 40m OCF antenna (as DJ0IP points out, a "single-core" 4:1 usually ain't good enough).The OP's proposal of using part of the coax shield as a counterpoise (and choking the current beyond it), is in principle similar to the resonant feedline antennas, T2LT, flower pots, and other variants.https://www.m0mcx.co.uk/m0mcx-banana-antenna-an-end-fed-choke-sleeve-resonant-feedline-t2lt-antenna-design/Incidentally for the OP, the above link is not an antenna to suit your needs (limited space for 80m), but it may give you an idea of how your 1:1 choke balun should be positioned with respect to your G7FEK antenna.

author: A.Z.
author_rank: Super Contributor
timestamp: October 30, 2023, 02:04:54 pm
content: well, if the main points are NVIS and getting down to 80m an aperiodic antenna built using just a run of coax may fit; pick about 21m (69ft) of coax, RG8 will do, shorten one end (solder center and braid together), connect the other end to a SO239 but wiring it "in reverse", that is coax braid to the SO239 center pin and coax center conductor to the SO239 "ground", lay out the resulting coax antenna as an inverted V, connect it to the coax feedline going to the shack adding a decent choke at the SO239 and there you go, use an ATU to adapt the impedance, the resulting antenna will work from at least 10 meters down to 80 meters, but don't expect miracles