Fan Dipoles, Baluns and SWR (oh my)
New Ham here, and this weekend assembled my first antenna, a fan dipole, for 40/20/10/6 meters, mounted in an inverted V beginning about 22 feet above ground level, fed by a length of 50 ohm coax into a 4:1 balun. I don't expect stellar performance from an antenna this low on 40 or 20m; adequate would be fine.
(Right off the bat I see something I did wrong; I used some plexiglass spacers to keep the wires separated, but the spacers are much too small (about 1.5" square), so I'm sure to be getting some mutual inductance across them, probably making the wires appear electrically a lot longer than they are.)
My question (once I've sorted out that spacing problem) is about tuning. I'm currently getting really terrible SWR according to the meter on my radio, going basically to infinity when modulating. My connections check out OK, so I suspect this is due to a pretty bad impedance mismatch. I used a 4:1 balun to hang the dipole, but now I wonder if 1:1 might have been better, given how low to the ground the antenna is for the longer bands.
How does one typically choose the ratio for a balun? And when adjusting the lengths of the wires on a long dipole with an antenna analyzer, since you can't be at both ends at once, what's the usual procedure for shortening the wires?
Sorry if these are total newbie questions... but then I am a total newbie
Whilst the fan dipole might be quite suited to your application, the knowledge gained in setting up a simpler single band half wave dipole with an effective 1:1 current balun and coax feed will be invaluable in setting up the more complex fan dipole.
Originally Posted by KJ6MQK
As noted already, you should be not be using a 4:1 balun in this application, my recommendation is a 1:1 current balun with high choking impedance.
Too much coupling between the wires of a fan dipole can make them very difficult to adjust because of the interaction between elements.
Try just the longest dipole with 1:1 current balun and learn how to tune its length for minimum VSWR. The notes at Optimising a coax fed half wave dipole should be helpful.
You mentioned an analyser, some clues are given at In pursuit of dipole resonance with an MFJ259B .
Do some googling and follow the specifications and the layout of hams who did this years ago. There are some because in the past I've looked at them. This will prevent either your hair going gray, or impending baldness
You have some missunderstandings as stated in your post.
A dipole normally has an impedence between 50 and 75 ohms depending on height.
That you have it mounted as an inverted V adds some other effects from the angle and the ends being nearer to ground.
The closeness of each band element has little effect.
Some fan types are made from 4 and 5 conductor old time rotor ribbon cable and work fne for the intended use.
You don't need a balun in this case unless you are intent to use one. It would be a 1 to 1.
If you can make the antenna install a flat type and match down the 40m band first, then each higher band as you go. Leave some means of final length trims.
What you should find is that the bandwidth will narrow on each band as you add the higher bands due to the extra capacity to "ground" (Q rises) and not to each other element, so the seperation has little effect between them.
I use a fan flat top cut for 75 and 40m with 5" spacing. With a tuner either internal or external, my antenna will work on nearly every band including 6m without any added elements. I can force it to work on 160 as well.
It's use is only on 75 and 40m and does really well with DX and local work at only 30 feet above ground on a mountain side. (it's not supposed to work that well at low heights but it does).
Other things you should condsider is dropping the feedline straight to the ground from the center then toward the radio location.
At some point make the feedline into a choke coil with about 10 turns in a 8 or 10" diameter and taped into a good coil form and not just 'hanked' up in a bunch. This will help block any common mode current from returning back to the radio when using the antenna toward the edges of it's 'reasonable' band width of about 2.5 to 1 upper and lower limits.
Tune the final results for the parts of the bands you most use.
These are the results and experience I have with my installation for only two intended bands. The (20, 15, 10) beam, 6 and 2m are beams on 2 towers.
Crikey, nobody mentioned the danger of baldness when I started getting into this. Should I invest in some Minoxidil now and save the trouble later?
I had done some Googling around, but I must not have been constructing the right query. Thanks for the advice; I shall surely take it, and probably drop by HRO tomorrow and pick up a 1:1 balun and an MFJ259B. I almost did when I was there spending my tax refund on Friday, but thought I'd see if I could make things work "good enough" without spending the money.
Really appreciate the feedback, and I'll post back to let you all know how it goes, lessons learned, etc
A pretty foolproof parallel dipole that covers 80-40-20-15-10 meters is the Alpha-Delta model DX-CC. It's pre-cut and pre-tuned and the wires are spaced properly so that usually if you just install it, it will be fine without doing anything more to it. On 80m it uses loading coils to shorten it a bit, so its bandwidth is not "the whole band" on 80: But it will cover 100-150 kHz of 80m pretty well, and you can select the part of the band you want to use (generally, phone or CW). On the other bands, it covers edge to edge.
To get on the air pretty quickly and easily, it's not a bad choice and it's made from very high quality materials.
For 6m, an inverted vee is kind of a loser (my perspective, as someone who's been active on 6m for 43 years now) and you'd do way better with a well elevated loop (omni) much higher than 22' above ground, or better still a rotary dipole. Problem with an inverted vee at 22' is it will work in two directions, and from SFO there aren't any two directions that are the best ones; 22' is also very low considering your height above sea level there is probably close to nothing. It would be a "very local" antenna that might make a few contacts around the Bay Area, and likely not much else. A rotary dipole at 40-50' would run rings around a fixed inverted vee at 22'.
Better still, a loop antenna on the car and a drive to Telegraph Hill or any number of well elevated and accessible locations will blow away anything you could do from home -- but that's true for most of us (me, too).
I agree with Owen that it's much easier, and probably more instructive, to start with a single-band dipole and become familiar with how to tune it before making something more complex like a parallel dipole. BTW with balanced antennas (dipoles) when you make adjustments, you don't need to be at both ends of the antenna at once -- you can calculate how much to adjust based on data from the analyzer, then make an equal adjustment at both ends. If the data indicates you need to shorten the antenna 10%, you remove 5% from each end to keep it balanced. That's simple. Getting a feel for what to do, exactly, takes some experience.