Doublet Dilemma: Unbalanced-L vs. Balanced Tuner - Page 1
Forum section: electronics / rf-microwave

author: ac8vk
author_rank: Contributor
timestamp: February 07, 2021, 07:26:22 pm
content: One of the more successful antennas that I have had at my semi-restricted QTH is an 84 foot wide doublet, feed with 300 ohm window line suspected about 33 feet up. This antenna is matched with a Icom AH-4 antenna tuner at the base of the 33 feet of feed line (coax and control line run back to the shack).One leg of the doublet is fed using the AH-4 long wire/whip output.The other leg is connected to the AH-4 ground connector, that is also grounded to the metal rod supporting the AH-4.Benefits of this antenna:• Operates on 80m to 10m, 20m performance seems much better than 40m.• AH-4 mates with my IC-7300 for near idiot proof operation.• AH-4 tunes quickly and emits very little power (0.3W) during tuning(I like not being one of those operators emitting a CW tone for all to hear while tuning up.)• The SWR on the coax is always under 1.5:1 - so the loss in the coax is as low as it can get.• Fairly stealthy - only folks who are in my back yard ever notice it.• AH-4 is well constructed.My sense is that this is noisy on receive.I am wonder if I am getting the most of the doublet - and I am looking for project for the shack.My question to the group:What if any improvement could I expect from a balanced tuner?The literature on doublets consistently emphasizes the need for a balanced tuner (such as a Double-L or S-match). The only specific problem they point to are challenges with using a balun on the tuned side of the L-network - whip tuners like the AH-4 don't have that balun.There are some interesting projects I could emulate (such as K3NG tuner, and PA0FRI designs) but I am left wondering if I can actually beat the AH-4. End up with a higher efficiently (i.e. more power into the Doublet watt for watt).At any rate, I would love to hear idea for the group.73,Erik / AC8VK

author: A.Z.
author_rank: Super Contributor
timestamp: February 08, 2021, 10:21:57 am
content: Quote from: ac8vk on February 07, 2021, 07:26:22 pmOff the top of my mind, the advantages should be an "undistorted" radiation pattern and possibly better matching, my humble suggestion is to try adding a balun between the ladder line and the AH-4 tuner, such a balun may be a 1:1 or a 4:1 one, example designs may be the following oneshttps://vk6ysf.com/balun_guanella_current_1-4.htmhttps://vk6ysf.com/balun_guanella_current_1-1.htmbut you may find other designs online, in either case, just enclose the balun in a weatherproof box and connect it between the ladder line and the ATU input (ant/gnd), also notice that the 1:1 may also be connected between the ATU and the coax going to the shack to minimize Common Mode currents (noise); as for your antenna, the doublet is a good one, but willing to extend its coverage on the various Ham bands, you may also consider the idea of turning it into a linear loaded doublet like the one shown herehttp://www.m0pzt.com/80m-cobra-doublet/Forgot, as for the BalUn I'd suggest you to start with a 1:1, then if you want, you may experiment the 4:1 but I believe that the 1:1 will work better in your case, the reason for this suggestion is explained herehttp://www.karinya.net/g3txq/tuner_balun/better than I could ever explain it

author: iMo
author_rank: Super Contributor
timestamp: February 08, 2021, 10:26:25 am
content: With one leg of the doublet grounded (as in your current wiring) the radiation pattern will not be symmetrical against the "ground", imho. Unless you can measure the radiation pattern of your doublet you may not notice it..

author: A.Z.
author_rank: Super Contributor
timestamp: February 08, 2021, 10:31:55 am
content: Quote from: imo on February 08, 2021, 10:26:25 amExactly, so putting a BalUn between the ATU and the ladder line should improve things quite a bit

author: iMo
author_rank: Super Contributor
timestamp: February 08, 2021, 10:54:09 am
content: Also the "not grounded" part of your 300 ohm ladder/window feed line in your wiring starts to radiate - a "symmetrical ladder/window feed line" (loaded with a "symmetrical dipole")does notradiate. That will contribute to the weird pattern and other potential issues as well..

author: ac8vk
author_rank: Contributor
timestamp: February 13, 2021, 02:10:30 pm
content: Thanks for the feedback... you have both given me some things to think about.While I don't think I have a pattern problem, I could experiment with an RF transformer or balun on the transmit side.And there are some alternatives to the simple dublet, such as the cobra to think about. That has actaually triggered me to revisit the possibility of a delta or quad loop.73s and thanks again,Erik

author: A.Z.
author_rank: Super Contributor
timestamp: February 13, 2021, 02:16:44 pm
content: Eric, as for the balun, I believe it may be worth a try, it won't take so much winding and testing one, just place it between the ladder line and the ATU, optionally (but recommended) add a choke to the coax going from tha ATU to your shackAs for the antenna, the linear loaded doublet is pretty easy to build and may allow you to better "load" on the lower frequencies, even if you don't have room for a full size doubletThen, if you instead have enough room, consider the idea of putting up a skyloop antenna

author: ac8vk
author_rank: Contributor
timestamp: March 13, 2021, 10:21:58 pm
content: This has given be quite a bit to think about - thanks for the commetns an ideas.I am going to try a few different antennas in this location this year... the cobra is one idea, I also think I can fit a 1x2 rectegular full wave 40 min loop.In the past (at another QTH) I fed a fan dipole (Alpha Delta DX-EE) located in the attic of that location fed by the AH-4 and a short legnth of window line.That antenna worked really well on 40 to 6M. I have been tempted to try it at this QTH, ourdoors.Regarding the balun... I think I will do some experiments there also. My biggest concern is the efficenty of the balun on the high SWR side of the tuner. But I can certainly wire the balun in and do some low power A/B testing using the reverse beacon network (or simmilar) robotic spotting system to measure if the balun improves the effectivenes of the system.I do have chokes on the feedline and control line to the AH-4.73 and thanks again.Erik

author: donmr
author_rank: Regular Contributor
timestamp: March 15, 2021, 10:43:33 pm
content: You need to do one of these things:1) Change to a single ended antenna, not a doublet. That is what the AH-4 is designed for.or2) Put a balun after the AH-4. It needs to be able to handle the higher voltages that can occur there. DX engineering sells baluns for this.or3) Float the AH-4 from ground using chokes on both the RF and control input cables. I don't like this because it could put high voltages back into your radio if the chokes are not good enough but some tuner manufacturers recommend this.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: March 16, 2021, 04:47:33 pm
content: I use a homemade balun based on this Balun Designs one, I used a pair of FT240-43 cores. I used PTFE sleeving over each wire and added a high Voltage resistor across the SO239 socket to drain static.https://www.balundesigns.com/model-1161-1-1-atu-current-balun-3-54-mhz-5kw/

author: A.Z.
author_rank: Super Contributor
timestamp: March 18, 2021, 03:04:30 pm
content: Quote from: donmr on March 15, 2021, 10:43:33 pmYou are right, but the OP (ac8vk) apparently wants to keep using the AH-4, so what we were/are trying to suggest are ways to couple the AH-4 to his doublet (fed using 300 Ohm window line); now, the problem is that the AH-4 (ran some searches) is an asymmetric antenna tuner which was designed to be used with whips and, in general, endfed antennas, so "adapting" it to work with a balanced antenna won't probably give optimal results; that being said, a possible approach may be using an 1:1 transformer (galvanic isolation) between the AH-4 and the window line, the secondary of the transformer would then go to the window line (antenna) and the primary will be connected to the AH-4 antenna input and to the AH-4 ground, the latter should be then be connected to a ground rod, that would allow to keep the AH-4 grounded while keeping the doublet "properly" fed, again far from being a perfect setup, just trying to "match" the OP request

author: donmr
author_rank: Regular Contributor
timestamp: March 18, 2021, 05:52:26 pm
content: Quote from: A.Z. on March 18, 2021, 03:04:30 pmThis what I meant by #2. What has worked for me is the DX Engineering product or 10-14 bifilar turns on a #31 or #43 240 size core. Use good high voltage wire with one connected to the tuner output and one to ground. A good ground is important too.

author: A.Z.
author_rank: Super Contributor
timestamp: March 18, 2021, 06:09:43 pm
content: I doubt that balun is a transformer offering galvanic isolation, would you please be so kind to post a schematic ?EDITjust in case I'm referring to something likethe above is a 9:1 but I think it clears the point