In years past, one of my favorite performers has been the venerable 134′ doublet. While the dipole is easy to construct, some issues and compromises emerge concerning various lengths of the 450Ω window line. I have spent some time and considerable “conventional” research on the matter to determine what would be optimal for me. On a lark, I decided to see what ChatGPT might suggest after seeeing what Google’s AI engine was tossing back at me.
For the most part, I do not use AI much more than it is embedded in the functionality of software now–and most of that I ignore or remove from my OS. I have tinkered with ChatGPT and Claude–mostly to see what all of this AI hoopla was about, and familiarize myself with how to make good queries. Now it seems to know a lot more about me, what I am looking for, and how to be conversational with me.
Below is the full transcript of my discussion with the model. I am rather impressed with this digital Elmer, to say the least and construction has already begun…
CHAT WITH GPT BEGINS
OPENING QUESTION
⇒ I would like to know the performance of a 134 foot dipole, 115 feet 450 ohm window line, 1:1 current balun to 10 foot lmr-240 coax, ldg at-100pro autotuner, at 50′ elevation
What you have is essentially a classic **134-foot doublet**:
* 134′ total wire length (67′ per side)
* 115′ of 450 Ω window line
* 1:1 current balun
* 10′ LMR-240 coax
* LDG AT-100Pro autotuner
* Height: 50 feet
Overall, this is a very capable all-band HF antenna system. The ladder line does most of the heavy lifting by keeping feedline losses extremely low even when the antenna is far from resonance. ([unicomradio.com][1])
### General Performance
At 50 feet high, the antenna should work well from 80 through 10 meters:
| Band | Performance |
| —- | —————————————— |
| 80m | Good regional coverage, some DX at night |
| 40m | Excellent all-around band, regional and DX |
| 30m | Very good |
| 20m | Excellent DX potential |
| 17m | Excellent |
| 15m | Excellent |
| 12m | Good to excellent |
| 10m | Excellent when open |
The 134-foot length is close to the traditional 135-foot doublet that many operators consider one of the best “one antenna” HF solutions. ([unicomradio.com][1])
### Height Effects
At 50 feet:
* On 80m you’re only about 0.19 λ high, so you’ll have a fairly high takeoff angle and strong regional/NVIS coverage.
* On 40m you’re about 0.38 λ high, giving a mix of regional and DX performance.
* On 20m you’re about 0.75 λ high, which is excellent for DX.
* On 15m and above you’ll develop multiple lobes and nulls, producing significant gain in favored directions. ([unicomradio.com][1])
### Feedline Length
One thing that caught my eye is the **115-foot window-line length**.
For an 80m-sized doublet, recommended ladder-line lengths often include approximately:
* 90–102 ft
* 160–172 ft
while about 135 ft is a length many builders try to avoid because it can transform impedances into values difficult for some tuners to match. Your 115-foot length is not in the commonly cited “problem” zone, so it should generally behave reasonably. ([balundesigns.com][2])
### Tuner Considerations
The LDG AT-100Pro is a good tuner, but it is not as wide-ranging as a large manual balanced tuner.
You may find:
* Easy tuning on 40–10m.
* Some challenging spots on 80m depending on exact wire length, installation, and feedline routing.
* Certain frequencies where the tuner simply cannot find a match.
That is normal for doublets because the impedance seen at the tuner can vary dramatically with frequency and feedline length. ([practicalantennas.com][3])
### Estimated Efficiency
Assuming:
* Window line is routed correctly and kept away from metal.
* Current balun is a quality design.
* Only 10 feet of coax.
I’d expect feedline efficiency on most bands to be very high:
| Band | Estimated System Efficiency |
| —— | ————————— |
| 80m | 85–95% |
| 40m | 90–98% |
| 20m | 95–99% |
| 17–10m | 90–99% |
The ladder line contributes very little loss compared to a coax-fed multiband dipole. ([DOLPH MICROWAVE][4])
### My Assessment
For a single-wire HF antenna at 50 feet, I’d rate this setup:
* **80m:** 7/10
* **40m:** 9/10
* **30m:** 9/10
* **20m:** 10/10
* **17m:** 10/10
* **15m:** 9/10
* **12m:** 8/10
* **10m:** 9/10
The biggest limitation is likely not the antenna itself, but whether the AT-100Pro can successfully match every band/frequency combination. The antenna and feedline arrangement are fundamentally excellent and should outperform many G5RV-style installations due to the longer ladder line and very short coax section. ([unicomradio.com][1])
If you tell me:
* Flat-top or inverted-V?
* Height at center and ends?
* Which bands you want to use most?
* Typical power level (100 W or less, since the AT-100Pro is rated around that range)
I can give a more detailed prediction of radiation patterns and likely gain on each band.
[1]: https://unicomradio.com/what-is-the-efficiency-of-a-doublet-antenna/?utm_source=chatgpt.com “What Is the Efficiency of a Doublet Antenna? – Unicom Radio”
[2]: https://www.balundesigns.com/reference/antenna-matching-problems-when-using-ladder-line-or-open-wire-feedlines/?utm_source=chatgpt.com “Antenna Matching Problems When Using Ladder Line or Open Wire Feedlines – Balun Designs”
[3]: https://practicalantennas.com/designs/dipole/multiband/?utm_source=chatgpt.com “multiband dipoles – Practical Antennas”
[4]: https://dolphmicrowave.com/default/what-are-the-different-types-of-antenna-feed-line/?utm_source=chatgpt.com “What are the different types of antenna feed line – DOLPH MICROWAVE”
⇒ alright then, what if I increased the dipole length to 135′?
Increasing the antenna from 134 feet to 135 feet will make essentially no practical difference in performance.
You’re talking about a change of:
- 1 foot total length
- 0.5 foot added to each side
- Less than 1% increase in length
For comparison:
|
Band |
Wavelength | 1-foot change |
| 80m (3.75 MHz) | ~262 ft | 0.4% λ |
| 40m (7.15 MHz) | ~131 ft | 0.8% λ |
| 20m (14.2 MHz) | ~65.5 ft | 1.5% λ |
In practice:
- Radiation pattern remains essentially unchanged.
- Gain remains unchanged (differences measured in hundredths of a dB).
- Takeoff angles remain unchanged.
- Feedpoint impedance changes only slightly.
- Tuner behavior will be nearly identical.
One Possible Benefit
A 135-foot doublet is a very common “standard” length because it’s close to a half-wave on 80 meters around 3.5–3.6 MHz. Some operators have found that certain combinations of:
- 135′ antenna
- specific ladder-line lengths
produce more favorable impedances on amateur bands.
However, the effect is mostly due to the interaction of antenna length and feedline length, not because 135′ is inherently better than 134′.
If You Want a Meaningful Change
Instead of adding 1 foot, consider:
- 140–145 feet → slightly stronger low-band performance on 80m.
- 160 feet → begins to improve 80m efficiency and can be used on 160m with a tuner.
- Raise the antenna from 50′ to 70–80′ → much bigger improvement than adding wire.
- Use a higher-power/wider-range tuner if you’re having matching issues.
What I’d Do
If your antenna is already cut and working:
Leave it at 134 feet.
The difference between 134′ and 135′ is so small that you would never be able to measure it on the air. If you’re going to put effort into modifying the system, height, feedline routing, or tuner capability will yield much larger improvements than one additional foot of wire.
what adjustment would improve characteristics at the upper end of 75 meters
If your goal is specifically to improve operation around 3.9–4.0 MHz, there are several possible adjustments, but they don’t all have the same impact.
- Shorten the antenna slightly (most direct)
A 134′ doublet is electrically a little long at the upper end of 75 meters. Shortening it a few feet moves its resonant region upward in frequency.
For example:
- 134′ → current configuration
- 130–132′ → slightly favors the upper portion of 75m
- 125–128′ → shifts the “sweet spot” even higher
The tradeoff is slightly less favorable behavior near 3.5 MHz.
- Change the ladder-line length (often more effective)
With a doublet, the feedline length frequently has a larger effect on tuner friendliness than a small antenna-length change.
Your 115′ of 450 Ω line transforms the feedpoint impedance to whatever the tuner sees. Changing the line by 10–20 feet can move difficult impedance points away from desired frequencies.
Common experimental lengths include:
- ~95–100′
- ~125–130′
- ~145–160′
A change of feedline length can turn a difficult 3.95 MHz match into an easy one without noticeably affecting radiation performance.
- Increase height (best for radiation performance)
If you’re concerned with signal quality rather than tuner matching:
- 50′ → about 0.19 λ at 75m
- 70′ → about 0.27 λ
- 90′ → about 0.35 λ
Raising the antenna lowers the takeoff angle and generally improves DX performance on 75m. It won’t necessarily make tuning easier, but it can make your signal stronger where it counts.
- Improve the tuner system
Many doublet users eventually move to:
- A wider-range manual tuner, or
- A balanced tuner
because some automatic tuners simply run out of matching range when confronted with the extreme impedances that ladder-line-fed antennas can present.
For your exact setup
If the antenna already tunes well across most of 80/75m and you’re only trying to improve the 3.9–4.0 MHz region, my order of experimentation would be:
- Try adding or subtracting 10–20 feet of window line (largest effect on matchability).
- If rebuilding the antenna is easy, shorten it by 2–4 feet total.
- Raise the antenna if possible.
A 1-foot change (134′ → 135′ or vice versa) is too small to materially improve the upper end of 75m. A 5–10 foot change in antenna length or a 10–20 foot change in ladder-line length is where you start seeing noticeable differences in impedance and tuner behavior.
⇒ So would i shorten or lengthen the ladder line to accomplish this, and would a smaller increment such as 5 feet make an impact as say 3.9mhz?
That’s the tricky part: without modeling or measuring, nobody can say with confidence whether shortening or lengthening the ladder line will improve 3.9 MHz specifically.
For a doublet, the ladder line is acting as an impedance transformer. At 3.9 MHz, the impedance seen at the antenna feedpoint is transformed by 115′ of 450 Ω line into whatever the tuner sees. Adding or subtracting line rotates that impedance around the Smith chart.
The electrical wavelength on typical 450 Ω window line (velocity factor ≈ 0.90–0.95) at 3.9 MHz is roughly:
- Free-space λ ≈ 252 ft
- In line ≈ 227–239 ft
Therefore:
- 5 ft is about 7–8° of electrical phase shift.
- 10 ft is about 15°.
- 20 ft is about 30°.
So yes, a 5-foot change can absolutely make a noticeable difference at 3.9 MHz. Whether it helps or hurts depends entirely on where your present impedance happens to be.
If it were my station
Since you’re at 115 ft now, I’d try:
- Add 5 ft (go to 120 ft)
- If that doesn’t help, try subtracting 5 ft (go to 110 ft)
A 5-foot experiment is large enough to move the impedance significantly but small enough that you’re not rebuilding the whole feedline.
A practical clue
What happens now at 3.9 MHz?
- Does the AT-100Pro fail to find a match?
- Does it tune, but with high SWR on the coax?
- Does it tune but performance seems poor?
- Is there a narrow “dead zone” (for example, 3.88–3.93 MHz) where the tuner struggles?
The exact symptom gives clues about whether you’re encountering a very high impedance, very low impedance, or high reactive component at the tuner.
Best method
If you have access to an analyzer such as an MFJ-259/269, RigExpert, NanoVNA, etc.:
- Disconnect the tuner.
- Measure the impedance at the balun/coax junction on 3.9 MHz.
- Note the R and X values.
With those numbers, I can make a much more educated guess whether adding or
subtracting ladder line is likely to move the impedance toward a tuner-friendly region.
For a 134′ doublet at 50′, my intuition is that a 5–10 foot change in ladder-line length is more likely to affect tunability at 3.9 MHz than changing the antenna from 134′ to 135′ or even 130′. The feedline length is the stronger lever in this particular system.
⇒ alright then, looking at the original configuration specs, what would the overall performance be if the window line were 120′?
If you keep everything else the same:
- 134′ dipole
- 120′ of 450 Ω window line (instead of 115′)
- 1:1 current balun
- 10′ LMR-240
- LDG AT-100Pro
- 50′ high
then the radiation performance of the antenna itself is essentially unchanged.
The antenna’s gain, takeoff angle, pattern, and efficiency are determined primarily by:
- antenna length,
- height,
- ground conditions,
- geometry (flat-top vs. inverted-V).
Changing the ladder line from 115′ to 120′ does not materially change those characteristics.
What does change?
The impedance presented to the tuner.
At 3.9 MHz, adding 5′ of line shifts the transformed impedance by roughly 7–8 electrical degrees. That’s enough to move a difficult tuning point noticeably.
You might see:
| Band | 115′ Line | 120′ Line |
| 80/75m | Good | Good to Excellent |
| 40m | Excellent | Excellent |
| 30m | Excellent | Excellent |
| 20m | Excellent | Excellent |
| 17m | Excellent | Excellent |
| 15m | Excellent | Very Good to Excellent |
| 12m | Good | Good |
| 10m | Good to Excellent | Good to Excellent |
The tradeoff is that when you improve one frequency region, another may become slightly less favorable.
Specifically for 75 meters
My suspicion is that a move from 115′ to 120′ would likely improve tuner friendliness in the upper portion of 75m (3.8–4.0 MHz) more often than it would hurt it, because 115′ is not a particularly “special” length and you’re only making a modest shift.
I would not expect:
- stronger signals,
- lower takeoff angle,
- more gain,
but I would expect:
- different SWR presented to the tuner,
- potentially easier matching near 3.9 MHz,
- possibly a broader range over which the AT-100Pro can find a match.
If this were my station
I’d probably choose 120′ over 115′ if my primary operating interest was upper 75-meter SSB (3.8–4.0 MHz), simply because it’s a small change that has a reasonable chance of moving the impedance away from a troublesome region without creating major new problems elsewhere.
The ideal way to know would be to model the system in EZNEC, 4NEC2, or AutoEZ, but from decades of practical doublet experience, a 5-foot change in ladder line is exactly the kind of adjustment operators make when chasing a stubborn tuning spot on one band while keeping excellent multiband performance.
⇒ What would happen if i added another 1:1 current balun at the end of the coax where it transitions to the tuner itself?
Adding a second 1:1 current balun at the tuner end of the 10-foot coax is one of those things that can range from “slightly beneficial” to “completely unnecessary,” depending on what’s happening in your station.
Your current arrangement is:
134′ doublet → 120′ (or 115′) window line → 1:1 current balun → 10′ coax → LDG AT-100Pro
If you add another current balun:
doublet → window line → current balun → 10′ coax → current balun → tuner
Potential Benefits
The second balun may further suppress common-mode current flowing on the outside of the coax shield.
Symptoms that suggest this might help include:
- RF in the shack
- Microphone bite
- Computer or audio interference
- Unstable tuner behavior
- Feedline radiation causing pattern distortion
In that case, a second choke at the tuner end can isolate the tuner and station equipment from currents on the coax shield.
What it will NOT do
It will not:
- Increase antenna gain
- Lower SWR at the antenna
- Improve radiation angle
- Make the doublet more efficient
- Create additional tuner matching range
Many hams expect an extra balun to improve matching, but that’s generally not how it works.
Possible Downsides
A current balun operating under severe mismatch conditions can see very high voltages and currents.
With a ladder-line-fed doublet, the impedance presented to the first balun can vary wildly across bands. Adding another ferrite device in the system means:
- More insertion loss (usually small)
- More heating potential at high power
- Another component that can saturate or become lossy
At 100 watts with an AT-100Pro, this usually isn’t a major concern, but it’s worth mentioning.
What I’d Recommend Instead
If your goal is common-mode suppression, I’d actually prefer a good coax choke at the tuner end rather than another traditional current balun.
Examples:
- 10–15 turns of the coax through large ferrite cores
- A purpose-built 1:1 choke designed specifically for common-mode suppression
This often provides better suppression than simply stacking baluns.
For Your Specific System
With:
- only 10 feet of coax,
- a current balun already installed at the ladder-line transition,
- a balanced doublet,
I would expect little to no noticeable change from adding a second 1:1 current balun unless you’re currently experiencing RF-in-the-shack symptoms.
If everything tunes properly and you have no RF issues, I’d spend my effort on:
- Optimizing ladder-line length,
- Keeping the window line well clear of metal,
- Maximizing antenna height,
before adding another balun.
One question: what model is the existing 1:1 current balun (ferrite core, Guanella design, commercial model, etc.)? The answer affects whether a second choke is likely to provide any benefit.
⇒ The balun is supposedly a 2000w ferrite made by bh7jyr
If it’s one of the BH7JYR 1:1 ferrite current baluns commonly sold online, then the key question isn’t the advertised 2000 W rating—it’s how well it maintains common-mode impedance across HF and how it behaves under highly reactive loads.
For your system:
134′ doublet → 115–120′ window line → BH7JYR 1:1 current balun → 10′ coax → LDG AT-100Pro
I’d expect the balun to work reasonably well, especially at 100 W, but a ladder-line-fed doublet can present some pretty extreme impedances on certain bands. That’s where balun quality matters most.
⇒ Would a second choke help?
Probably not much.
The first balun is already located where it does the most good:
- At the transition from balanced line to coax.
- Where common-mode currents are most likely to be induced.
Adding a second identical balun only 10 feet away is often redundant.
One exception
If you’re seeing any of these:
- RF feedback in the shack
- Hot microphone
- RF affecting computers or audio gear
- Coax shield becoming part of the antenna
then a choke at the tuner end can sometimes help.
In that case, I’d prefer a dedicated common-mode choke rather than a second voltage/current balun.
What I would be more interested in
For your antenna, I’d be curious about the tuner’s behavior on:
- 3.9 MHz
- 7.2 MHz
- 14.2 MHz
- 28.4 MHz
If the tuner matches all of those quickly and repeatably, then the BH7JYR balun is probably doing its job adequately.
One thing to watch
Many commercial “2000 W” baluns are rated assuming:
- Near-50 Ω loads,
- Low SWR,
- SSB duty cycle.
A 134′ doublet with ladder line can present transformed impedances of several hundred or even several thousand ohms on some frequencies. That stresses the balun much more than a resonant dipole would.
So I’d be more confident in saying:
The balun is probably fine at 100 W with your AT-100Pro, but I would not assume the 2000 W rating applies under every mismatch condition that a doublet can produce.
My overall assessment
With your current setup, the biggest factors affecting performance are:
- Antenna height (50′)
- Ladder-line length (115′ vs. 120′)
- Feedline routing
- Tuner matching range
The balun is probably not the limiting factor right now.
If your primary interest is improving the upper end of 75 meters around 3.9 MHz, changing the ladder-line length by 5–10 feet is far more likely to produce a noticeable effect than adding a second 1:1 balun. If I were experimenting, I’d try 120′ of line first and observe how the tuner behaves at 3.9–4.0 MHz before making any other changes.
END OF TRANSCRIPT
I am certainly coming back to this conversation later, after I see how the tuner responds across the bands! More to come…
