Backstay antenna length decision

[MJH]

Originally Posted by s/v Jedi

With a good automatic antenna tuner you should be able to TX at all frequencies. My smart tuner (sgc works on every radio) does for sure.

That said, I would go down 2 meters from the top so 15m length. The reason is that I know it works well on both the SGC and Icon tuners I have. If you have a different one then you may have different requirements.

The Googled answer to the question is:

23 feet


Primarily, the antenna must be as long as practically possible, but certainly no shorter than 23 feet. This magic number is not arbitrary, it's the length of the shortest untuned vertical fiberglass whip antennas that might be used with an SSB transceiver.Jan 1, 2003

Setting up single sideband - Ocean Navigator - January ...

www.oceannavigator.com › January-February-2003 › Setting-up-single-side

~ ~ _/) ~ ~ MJH

[derfy]

Originally Posted by sailingchiro

Researched this subject last year and 27' was the general consensus. I will say the most important (found as a result of research) part and hardest part is the ground plane. As in Keep it simple as in the KISS device seems to work pretty well.
I don't use the SSB as much as thought. Many are going to phones with data.

Thx



Just curious, can you Tx to 4,6 MHz Marine or 80 or 60 Meter Ham? Seems a bit too short for those...



27' is perfect for 6,8 MHz Marine and 40 Meter Ham. Probably works for 12 MHz, 30 Meter and 20 Meter Ham with heavy tuning. Not a bad choice.

[continuouswave]

SHORT ANSWER: make the backstay insulated length plus the GTO feed wire length a total of 28.5 feet.

EXPLANATION:
A good empirical formula for calculating the resonance of a half-wavelength antenna made from typical wire diameters above a typical earth is

L-h = 468/F where L-h is in feet and F is in Mhz.

On that basis a quarter-wavelength can be estimated to be

L-q = 234/F where L-q is in feet and F is in Mhz

The advantage of having the end-fed antenna close to a quarter wavelength is the feedpoint impedance of the antenna will be low, the rate of change of impedance will be low, and obtaining a good match and good power transfer from a 50-Ohm transmission line will be easy to accomplish with a good antenna matching unit that can handle a range of load impedances that are not too high or too low with good efficiency.

The feedpoint impedance of an end-fed antenna will also be low at odd-multiples of one-quarter wavelength, so a three-quarter or five-quarter wavelength works.

When estimating the length of the antenna using a backstay, you should include the length of the high-voltage wire that carries the antenna to the antenna matching unit, as that becomes part of the antenna.

For use at 10-MHz, which you mention as perhaps the primary frequency, the length of a quarter-wavelength antenna would then be

L-q-10 = 234/10 = 23.4-feet

If the connecting wire between the backstay and the antenna matching unit is 5-feet long, the backstay length would be 18.4-feet.

Repeating this analysis for 8-MHz or 7-MHz would give the total antenna length to be

L-q-8 = 234/8 = 29.25-feet

L-q-7 = 234/7 = 33.4-feet

By feeding the antenna at a quarter-wavelength, there will be a current maximum at the feedpoint. This antenna current must have some path to return to the coaxial transmission line shield. The path is usually achieved by a ground plane. The ground plane can consist of insulated radials, or in the case of a boat at sea, particularly a metal-hull boat, by using the boat hull and its connection to the seawater as a ground plane. Boats with non-metal hulls usually have a dedicated electrode plate immersed in the sea or have large surface areas of the inside of the hull lined with copper straps to create a big surface area connecting to the seawater through the non-conducting hull, making a big distributed capacitor.

Feeding at a quarter-wavelength will require some sort of radials or ground plane effect in order to produce an efficient antenna.

If the boat has a metal hull or has some provisions for an effective ground plane with the seawater, an end-fed antenna with a total length (including GTO feed wire from the antenna matching unit) of 28.4-feet should be useful from 7 to 10-MHz as a low-impedance feedpoint end-fed antenna.

10-MHz length = 23.4
7-MHz length = 33.4
Average = (23.4 + 33.4)/2 = 56.8/2 = 28.4-feet

If the connecting wire feeder is 5-feet, make the backstay antenna 23.4-feet.

(ASIDE: I have no idea how the other fellows came up with the very similar lengths for the backstay antenna. I have explained my derivation.)

ALTERNATIVE:

Going the other way, an end fed antenna at half-wavelegth resonant length can be fed at a high-impedance impedance point, which produces a high voltage but much lower current. In this method, the antenna current can often be couple with a counterpoise arrangement. The counterpoise does not have to be large or resonant, because of the high impedance

The problem with end feed of a half-wavelength antenna is the feedpoint impedance will vary rapidly with frequency change, will be several thousand Ohms, and won't be a good match to a 50-Ohm transmission line.

While some antenna matching units may be able to indicate they have produced a good match to a very high antenna impedance by producing a low VSWR on the 50-Ohm transmission line, there is often significant power lost in the matching unit itself. Obtaining a match with efficient transfer of power from a 50-Ohm transmission line to an end-fed half-wavelength will be difficult.Therefore, as a general rule, end feeding of an antenna that is naturally resonant at a half-wavelength should be avoided.

The high feedpoint impedance of end fed half-wavelength resonant antennas is often matched to 50-Ohms by using a tapped coil in a transformer arrangement. The design of a proper coil, proper inductance, and location of the tap are often done empirically. And the matching properties become very frequency sensitive. While a reasonable approach for a dedicated length and frequency, the tapped coil won't be very useful with multiple frequencies.

de K8SS

[derfy]

An excellent analysis.

I'm shooting for a quarter wave end feed.

The purpose of my thread was to find out what the best target frequencies really are. 10 megahertz is not one of them.

My back stay is long enough to build a quarter wave 4 megahertz antenna. 80 meters. If I want to.

but there are a lot of important cruising nets in the 6, 7, and 8 megahertz region. This includes the 40 meter ham band.

As you point out. A quarter wave antenna at 80 meters won't work very well at all at 40 meters, where it is a half wavelength.

I can probably build a long antenna that's good at 80 meters (4 megahertz) or a shorter antenna that does well at 40 meters (7 megahertz). It's hard to build one that'll do both.

The question is, which is better?

[ka4wja]

derfy,
FYI, much of this info is in the "stickies" right at the top of the "Marine Electronics" page here on Cruiser's Forum....

1) The really short answer is:
Make it long....don't worry about its exact length...

The defacto "standard" for sailboat backstay antenna lengths has been (for the past 30+ years) 40' - 45'....although this was always a compromise and was based on higher sunspot numbers overall (which we do NOT have anymore), and a lack of RFI generating crap on-board (which we DO have now), so this old "standard" was making assumptions on old facts...
Now-a-days, the "standard" is generally: make it longer...40' to 60'...
Optimize it for 4mhz thru 8mhz....these will be the most critical....(and on up thru 12mhz, but much less critical)....
As for ham-band "optimals", they are actually a bit different....they would be 40m, with 20m a close second place, and then 75m...


2) The sorta' short answer is:
Make it long....don't worry about its exact length...
As long as it is between ~ 20' and ~ 85', your remote tuner (Icom, SEA, or SGC) should couple/match your transceiver to it, without issue....(no matter what the manual says, nor what well-meaning advice you've gotten)....decent remote tuners can couple some rather odd loads to the transmitter...(generally longer is better...lower tuner losses, esp. on the lower freqs....and more effective/efficient radiation)

And, using your info here (56'), the short answer is....
Place the upper insulator about 4' to 6' from the masthead, and you'll have an approx 50' - 52' long antenna, which will allow effective tuning and very efficient radiation, and a decent low-angle of radiation, from 1.6mhz thru 12mhz....(and still work pretty darn well, all the way thru 30mhz)
If you really desire to opt for a low angle of radiation on 14mhz (or 16mhz, or 18mhz, or above) then you could certainly place the upper insulator down quite a bit further, thereby providing you with a shorter antenna, at the sacrifice of antenna efficiency (and higher tuner losses) on the lower bands...
{But, please note that the antenna starts at the tuner....not at the backstay....so, please add this into your decision...}

Understand that generally longer (in terms of wavelength, of course) is better (more efficient radiator, and lower tuner losses) than shorter....especially true for the low end of the HF region.
On our fairly small boats, the lower HF and upper MF, from 1.6mhz thru 8mhz, or so, radiator in-efficiency and especially higher tuner losses are particularly problematic....
Hence why we say "longer is generally better".
Please allow me to repeat that....
On our fairly small boats, the lower HF and upper MF, from 1.6mhz thru 8mhz, or so, radiator in-efficiency and especially higher tuner losses are particularly problematic....
Hence why we say "longer is generally better".

{Fyi, anecdotal info and simple math, have confirmed that typical comms with a 23' whip, compared to a 45' long backstay, on 4mhz thru 8mhz, those with the shorter antenna are significantly worse off....by as much as 6db to 10db....as the length differences get wider and the freq goes lower, the differences are greater....and always longer is better!
And, if you have an older or poorer designed tuner, these differences can be even greater!}

[BTW, on my current boat, my backstay antenna length is approx. 62'....as I desired to optimize for the lower HF bands....and I've been extremely pleased with these results for the past 15 years!]



3) Furthermore....
A slightly longer answer....all the above included, plus:

--- All HF maritime GMDSS comms is from 2mhz thru 16mhz.....with most 12mhz and below (these days 8mhz being daytime primary, and nighttime 4mhz being primary), and even though here in US (and in Aus, etc.) there aren't any MF coast stations, remember that 2mhz/MF is useful for SOLAS ship signaling at sea, etc....as well as quite useful in EU, UK, the Med, etc...

--- Most HF maritime "cruising" comms / routine HF maritime comms (cruising nets, weather broadcasts, WeFax, etc.) is from 4mhz thru 12mhz...(with 4mhz and 8mhz being the most popular)

--- These days (lower sunspot numbers) most HF ham communications is between 3.5mhz and 14.35mhz....(although I did make a 15m contact this summer!)

--- Most HF ham "sailing" communications is near 3.8mhz (local/regional nets), 7.2mhz (regional nets, and misc.), and 14.3mhz (long-range nets, and misc.)


And, when the upper HF bands (16mhz/18mhz and above) are "open" for comms, signals are generally good, which means that any antenna pattern deficiencies on these bands are usually not a problem...
It is on the lower HF bands, especially the bottom end of the spectrum, where antenna efficiencies and lower tuner losses become critical issues!


BTW, using your words of backstay antenna being 56' long overall, I assume that is all the way to the masthead? Not taking into account the usual "place upper insulator 4' - 6' from masthead"?
A Morgan 44's specs (E= 14.25', P=46.5', I= 53.2', J= 17.5') and with the backstay chain plate a couple feet aft of the boom...I'd make a guess that the backstay length is about 55.5' to 56'...

So...
Once you place the upper end of your antenna stop (upper insulator) approx. 4' to 6' from the masthead, you have a 50' to 52' long insulated section of backstay....and the lower end of your antenna starts right at the remote tuner output terminal....so, your antenna length includes the GTO-15 wire from the tuner to the backstay....



4) FYI....the old adage (many times included in remote tuner manuals) to "stay away from 1/2-wave lengths and multiples of 1/2-wave lengths" for end-fed antennas is based on one odd peculiarity that does not exist on our boats, and two "out-dated" issues:
a) an end-fed 1/2-wave antenna that is small in diameter (such as 14 ga copper wire) presents a very high impedance (2000 - 3000 ohms), but one that is made of 3/8" diameter has about 1/2 that feed impedance....
So, when feeding SS "backstays" these old cautions are usually moot!

b) many early design remote tuners had difficulty matching the very high impedances, that the more modern tuners (Icom At-130, AT-140, SGC-230, SEA 161, etc.) are capable of.

c) high-impedance end-fed antennas have very high RF voltages that can arc-over, etc. and/or cause transmit RFI / RF feedback into the older transmitters....


5) So, to sum up....
Make you backstay the length that you determine will provide you with the best overall performing antenna, for you application!

For your stated application, on your current boat, I'd go back to my "sorta' short answer above....
Place the upper insulator about 4' - 6' from your masthead, leaving you a 50' - 52' long antenna....
This will be good!


I hope this helps!

Fair winds and good luck (and if you wish to discuss this directly, send me a PM with your phone # and I will call you this evening.

John


P.S. EDIT....I started this posting earlier, before many of the responses...and had family matters pull me away....sorry about that.
Read what Nick (s/v Jedi) wrote...

[ka4wja]

Whoa there derfy...
1) I just saw this new info you posted...
I'm sorry I made an assumption your were asking about HF maritime communications?
My mistake...
You do not have a maritime HF rig (nor tuner) and are asking about ham radio freqs..

Again, I'm sorry for rambling on about HF maritime comms!

Originally Posted by derfy

My configuration...

Icom 718 mounted midship at nav station with 9-14vdc>13.8vdc regulated power supply. Chassis grounded to 12 V ground at the keel w 10 AWG stranded - 6 ft. Ferrite Core Filters on Power, tuner cable, and coax feeds.

Icom AH-4 tuner in aft lazarette just below deck at backstay chain plate

KISS ground counterpoise assembly at AH-4 in stern lazarette.
GTO-15 feed to cable above the lower backstay insulator (above my hydraulic backstay tensioner), about 6 ft, using standoffs from un-insulated backstay section.
Thx

Optimizing the length of your backstay antenna for HF ham radio operation is different than that for HF maritime comms....(yes, that's probably going to seem weird to you, since their bands are adjacent to each other throughout the HF spectrum....but it is more about how/who/where these communications are taking place, rather than specifically what freqs)

As for ham-band "optimization" of a backstay antenna length....look at 40m, with 20m a close second place, and then 75m...
So...

So, here I'd recommend placing the upper insulator quite a ways down from the masthead....
Perhaps limiting your overall antenna length to the old standard of 40' to 45'!

Sorry about all the above info in my earlier post (it's all great and accurate, but based on maritime HF comms, not ham radio HF comms), but do hope you do read it over, and send me a PM and I can call you this evening...



2) After you get the rigging work done, and you have the time to learn a bit more, please take note that you've really hemmed yourself in with a few choices you mention above (KISS, and an IC-718).

Please note that you'd be much better off with a direct sea-water connection for an HF antenna ground....or even if that is completely out-of-the-question, if your "KISS" is newly purchased, please return it.....you can make an artificial counterpoise that works better, for < $5!! And, you can use other things/items that will work even better, for free!
(or best overall to look at using a direct sea-water connection antenna ground!)

And, when using a ham radio on board (while fun for us hams) you've limited yourself to only HF ham comms....
Understand that no ham radio is "clean" enough to operate on the maritime bands/freqs (hence why it is illegal to do so), and even if you desire to interfere with others communications (and flaunt the law as well), you have no MF/HF-DSC signaling....and you might not be aware that for more than 20 years now it is MF/HF-DSC signaling that allows maritime comms (outside of VHF radio range) to take place....you know the 1000's of ships at sea monitor MF/HF-DSC 24/7/365 (not any voice channels), as well as > 80 HF-DSC coast stations worldwide, and >450 MF-DSC coast stations worldwide, all listening for DSC calls (not voice calls)....and once signaled via MF/HF-DSC, switch to a working SSB Voice channel....
But, without MF/HF-DSC, you have no way of contacting any of them.

Again, all of this (and much more) is covered in the stickies above...
But again, this isn't the question you asked....so no rambling on about it all here...



Fair winds...

John

[derfy]

Originally Posted by ka4wja

Whoa there derfy...
1) I just saw this new info you posted...
I'm sorry I made an assumption your were asking about HF maritime communications?
My mistake...
You do not have a maritime HF rig (nor tuner) and are asking about ham radio freqs..

Again, I'm sorry for rambling on about HF maritime comms!



Optimizing the length of your backstay antenna for HF ham radio operation is different than that for HF maritime comms....(yes, that's probably going to seem weird to you, since their bands are adjacent to each other throughout the HF spectrum....but it is more about how/who/where these communications are taking place, rather than specifically what freqs)

As for ham-band "optimization" of a backstay antenna length....look at 40m, with 20m a close second place, and then 75m...
So...

So, here I'd recommend placing the upper insulator quite a ways down from the masthead....
Perhaps limiting your overall antenna length to the old standard of 40' to 45'!

Sorry about all the above info in my earlier post (it's all great and accurate, but based on maritime HF comms, not ham radio HF comms), but do hope you do read it over, and send me a PM and I can call you this evening...



2) After you get the rigging work done, and you have the time to learn a bit more, please take note that you've really hemmed yourself in with a few choices you mention above (KISS, and an IC-718).

Please note that you'd be much better off with a direct sea-water connection for an HF antenna ground....or even if that is completely out-of-the-question, if your "KISS" is newly purchased, please return it.....you can make an artificial counterpoise that works better, for < $5!! And, you can use other things/items that will work even better, for free!
(or best overall to look at using a direct sea-water connection antenna ground!)

And, when using a ham radio on board (while fun for us hams) you've limited yourself to only HF ham comms....
Understand that no ham radio is "clean" enough to operate on the maritime bands/freqs (hence why it is illegal to do so), and even if you desire to interfere with others communications (and flaunt the law as well), you have no MF/HF-DSC signaling....and you might not be aware that for more than 20 years now it is MF/HF-DSC signaling that allows maritime comms (outside of VHF radio range) to take place....you know the 1000's of ships at sea monitor MF/HF-DSC 24/7/365 (not any voice channels), as well as > 80 HF-DSC coast stations worldwide, and >450 MF-DSC coast stations worldwide, all listening for DSC calls (not voice calls)....and once signaled via MF/HF-DSC, switch to a working SSB Voice channel....
But, without MF/HF-DSC, you have no way of contacting any of them.

Again, all of this (and much more) is covered in the stickies above...
But again, this isn't the question you asked....so no rambling on about it all here...



Fair winds...

John

Hi John


Thanks for this. I sent a private message.

[derfy]

Thanks to all for your prompt and helpful responses.


I will follow up this thread with a final report in a few weeks on what I ended up doing and how it has works out.


I will by trying out my new rig in the next couple of weeks on the Neuse River in NC, before heading offshore.



It would be greatly appreciated if some folks were able to help me evaluate my rig.



Please send a private message if you might be available to check my signal strength, etc, on the ham bands, particularly the 80 Meter, 40 Meter, and 20 Meter bands.


Thanks and Best Regards to All


derfy

[Boatguy30]

The tuner sees the lead wire and the backstay section as part of it's length. So it's generally pretty random unless you measure exactly. I usually put the top isolator 6' from the masthead and the lower above the Bimini or head height.

On my own boat I generally use a single wide hung inside a line. 33' or I think I currently have 23' 1/8" lifeline cable crimped direct to tuner lug.

[waterman46]

Originally Posted by derfy

My rigger is coming tomorrow to put in the new backstay! Opinions needed now, thanks in advance.


I am a newbee to marine and ham SSB, but have a ham General License, Marine FCC License, and I am a used-to-be electrical engineer. I am preparing my vessel for offshore and need voice and data SSB. US East Coast, Atlantic, and Caribbean destinations. I am replacing my backstay tomorrow, and have the option to make the insulated part up to ~56 ft (17 m), or arbitrarily shorter. This includes to GTO wire length from the tuner in the aft lazerette.


The quarter wave resonant freq should be 4.4 MHz w 56 ft, slightly above the 80 meter ham and 4 MHz Marine bands. My tuner docs warn not to transmit on the half wavelength frequency (8.8 Mz) to avoid output stage damage to the rig - makes sense.


At 56 ft, I should be able to tune and Tx on Marine SSB 4, 12, and 22 MHz, and Ham 80, 30, 20, 15, and maybe 10 meters.


I will not be able to Tx on Marine SSB bands 8, 16, 18, or 25 Mhz or Ham 40, 17, or 12 meter. I hope I can Rx these to some degree.


Iffy for Tx are are Marine band 6 MHz and 60 and 10 meter Ham.


WHERE THE TRAFFIC IS: I note that the Marine 8 MHz has the most nets, followed by Marine 6 MHz, and Ham 40 meters. The most Winlink gateways are on 40 meters. I will not be able to Tx to any of these.


DECISON: I am screwing up with such a long antenna? I could shorten it to, for example, 34 ft, centered in the 40 meter Ham band, and be able to Tx on Marine 6 MHz and 8 MHz.


I would lose Tx-ing on the 80, 20, and 10 meter Ham bands, and lose Marine bands 4, 6, 12, and 25 MHz. These have a lot of nets and Winlink gateways also.


Voice of Experience would be very welcome. Thx in advance.

Voice of Experience Here. Wow, you are seriously overthinking this thing. Why do you say you will not be able to transmit on so many frequencies you listed?

I am an electrical engineer too, and I know the temptation is to try to apply any and all knowledge you have, to such a thorny problem. But I have finally overcome that in this case because there are just too many unknown and unknowable variables.

In the first place, your tuner is capable of tuning almost anything not exactly on a half wavelength or a complete short. Don't worry - it will work very will on 95% of 2 to 30 MHz.

In the second place, there is no way for you to accurately compute the frequency of 1/2 wavelength presented to your tuner, given all the physical surroundings (the rest of the rig, the ground system, the capacitive coupling to the hull in the lead-in).

HOWEVER it is important to have as long a backstay antenna as you can manage, given that you want good output on the lower end of the HF band. Especially with the horrible sunspot cycle we now have. But keep the top of the antenna a bit away from the top of the mast, so put the upper insulator about 4 feet down from the top of the backstay. On the lower end, you don't want salt spray to short out the insulator, so keep that one about 8 feet above the waterline.

That's it, no hassle, just do it! BTW, pay close attention to the method used to attach antenna to backstay wire. Dis-similar metals, lots of weather, etc. I wrap it up in rigging tape but still don't trust it. Maybe someone on the forum can answer that issue.

[Topspin]

Dumb Question: As an alternative to the backstay antenna, why not spanning a rope antenna (or just an isolated wire) from the lateral edge of the stern arch to the top of the mast (with a pulley) and place the antenna tuner below the solar panels.
That would save the cost for isolators. OK, there may be interferences with the backstay, but it would make the length of the antenna way easier to adjust....
It would also remove the problem of a connection point between antenna wire and feed wire because the antenna wire could be connected directly to the tuner.


@John, KA4WJA: I would be highly interested in your concept of a <5$ counterpoise. would you please elaborate that a bit!


-Richard (VK4WRS)

[Brian.D]

John's info on a <$5 counterpoise system can be found here.

The alternate backstay antenna is the best solution. If you cut your backstay (and this might be too late) and it does not work for you, well, it was a waste of money. With an alternate backstay antenna there is no structual load changes and if you don't like the length, take it down and put up another. Easy.

As far as working ham bands, don't think of tuning in the band. Cut your antenna outside the band. Like, for 40 meters cut the antenna length to maybe 6.8MHz. But don't cut 1/4 wave in the 7~7.3 MHz.

Here are two links that are good reads for Auto antenna tuner and wire lengths, and for random length antennas.

https://g8jnj.webs.com/usingautotuners.htm - in this link the magic number is 23' and 30'.

Random Wire Antenna Lengths

I know I am probably too late, but maybe someone else will benefit from this data.

[derfy]

Originally Posted by waterman46

Voice of Experience Here. Wow, you are seriously overthinking this thing. Why do you say you will not be able to transmit on so many frequencies you listed?

I am an electrical engineer too, and I know the temptation is to try to apply any and all knowledge you have, to such a thorny problem. But I have finally overcome that in this case because there are just too many unknown and unknowable variables.

In the first place, your tuner is capable of tuning almost anything not exactly on a half wavelength or a complete short. Don't worry - it will work very will on 95% of 2 to 30 MHz.

In the second place, there is no way for you to accurately compute the frequency of 1/2 wavelength presented to your tuner, given all the physical surroundings (the rest of the rig, the ground system, the capacitive coupling to the hull in the lead-in).

HOWEVER it is important to have as long a backstay antenna as you can manage, given that you want good output on the lower end of the HF band. Especially with the horrible sunspot cycle we now have. But keep the top of the antenna a bit away from the top of the mast, so put the upper insulator about 4 feet down from the top of the backstay. On the lower end, you don't want salt spray to short out the insulator, so keep that one about 8 feet above the waterline.

That's it, no hassle, just do it! BTW, pay close attention to the method used to attach antenna to backstay wire. Dis-similar metals, lots of weather, etc. I wrap it up in rigging tape but still don't trust it. Maybe someone on the forum can answer that issue.

As stated ... I am a newbee ... what do I know? Just read on my tuner datasheet that is won't work well at 1/2 wave x N freqs, so, silly me, I calc'd the 1/2 wave freq, based on the backstay and GTO wire length, and it lands in the middle of 40 meter band, and Maritime 8 MHz.

[derfy]

Originally Posted by Brian.D

John's info on a <$5 counterpoise system can be found here.

The alternate backstay antenna is the best solution. If you cut your backstay (and this might be too late) and it does not work for you, well, it was a waste of money. With an alternate backstay antenna there is no structual load changes and if you don't like the length, take it down and put up another. Easy.

As far as working ham bands, don't think of tuning in the band. Cut your antenna outside the band. Like, for 40 meters cut the antenna length to maybe 6.8MHz. But don't cut 1/4 wave in the 7~7.3 MHz.

Here are two links that are good reads for Auto antenna tuner and wire lengths, and for random length antennas.

https://g8jnj.webs.com/usingautotuners.htm - in this link the magic number is 23' and 30'.

Random Wire Antenna Lengths

I know I am probably too late, but maybe someone else will benefit from this data.

Thanks Brian. Great reference articles.

Martin (your 1st article's author) suggests to avoid extreme feed point impedances (very low at 1/4 wave, very high at 1/2 wave), by choosing a length somewhat off center from targeted frequencies. That avoids extreme voltages and currents in the autotuner. Makes a lot of sense to me.

I cut the backstay for my max length (~56 ft to the upper insulator). Should be 1/4-wave resonant at 4.4 MHz - near (but somewhat above) the Ham 80 Meter band, and 1/2-wave resonant at ~8.8 MHz - pretty far north from the Ham 40 meter band, but in top of Maritime 8 MHz.

I suppose all of these calculations could be off 10% - in which case we might as well be contemplating the number of fairies that can dance on the head of pin?

Naively, I think my measurements might be better than 10%

Martin's article points out that shorter lengths will work best at higher freqs, and suggests some longer lengths also, 19.4 meters or longer, for lower freqs. Ham HF gurus advise me to go for max power at the lower freq end of things due to upcoming solar minimums.

Sounds reasonable ... what do I know? I am working on wiring and ground plane today. Will report back when I start test transmissions.

[s/v Jedi]

Your math is way off, much more than 10%. You ignored my comment on shortening factor earlier so let me try with a citation so you can learn this from Wikipedia (it’s called the velocity factor): https://en.wikipedia.org/wiki/Velocity_factor

Originally Posted by derfy

Thanks Brian. Great reference articles.

Martin (your 1st article's author) suggests to avoid extreme feed point impedances (very low at 1/4 wave, very high at 1/2 wave), by choosing a length somewhat off center from targeted frequencies. That avoids extreme voltages and currents in the autotuner. Makes a lot of sense to me.

I cut the backstay for my max length (~56 ft to the upper insulator). Should be 1/4-wave resonant at 4.4 MHz - near (but somewhat above) the Ham 80 Meter band, and 1/2-wave resonant at ~8.8 MHz - pretty far north from the Ham 40 meter band, but in top of Maritime 8 MHz.

I suppose all of these calculations could be off 10% - in which case we might as well be contemplating the number of fairies that can dance on the head of pin?

Naively, I think my measurements might be better than 10%

Martin's article points out that shorter lengths will work best at higher freqs, and suggests some longer lengths also, 19.4 meters or longer, for lower freqs. Ham HF gurus advise me to go for max power at the lower freq end of things due to upcoming solar minimums.

Sounds reasonable ... what do I know? I am working on wiring and ground plane today. Will report back when I start test transmissions.