The LARG HF Multiband Antenna Projects

From: Erik Werner  - KD5CTJ

Thursday, March 23, 2000     To: L_A_R_G Reflector

Simple Antenna Question:   Multi-trap dipole or centerfed multi-dipole and why?

Of course we're are talking HF - 10/15/20/40/80 Meter Bands

First de Paul - K4MSG

Erik: Assuming you intend to use coax feed:

1) Multitrap dipoles are expensive and the traps tend to be heavy since the construction is usually a coil wound over two concentric pieces of aluminum tubing (which forms the capacitor). But they do work.

2) A 5-band trap dipole (only one set of traps) is OK, but the antenna won't do well on the WARC bands even with a transmatch.

3) A G5RV is a reasonably decent antenna, albeit something of a compromise (I don't care what the sales literature says). But it will work 80-10 including WARC.

4) "Nested" multiple dipoles will work, but it is a more complicated structure to work with and requires a fair amount of "pruning" and checking band-to-band to get things right (because the dipoles will interact).

5) I use a W9INN HPD-3-125, which is a 125-foot dipole designed for 160, 80 and 40. It has two "resonactor" coils in each leg and at the design frequency in each band (I had John design it for the low-end CW portions of all three bands) the VSWR is 1.1 with no transmatch. Height above ground and trees will affect it, so a little fine tuning is usually necessary. I feed it with coax and use a transmatch and it will load on any band, including WARC, and works pretty well. It's absolutely great on 80 and 40 and even does a decent job up and down the East coast on 160 (for which you really should use a vertical with about a bazillion radials). Feed the W9INN with ladder line and use a transmatch and of course it will perform wonderfully on any band - but then, an ordinary dipole 50' or more long, when fed with ladder line and a transmatch, will be likewise. In fact, the latter is the simplest and most effective dipole you can have - but then, you either need the shack right next to a window with the transmatch on the window sill, or you have to very carefully route the ladder line into and through the house (it can be unbalanced by metal gutters, wiring in the house walls, etc.).

Hope this helps. 73, Paul, K4MSG

Second de Bill - K8SYH

Hi Erik, Well, your best choice (if you could do it) would be a single 80 meter dipole fed with balanced feed. That would give you what you want. Since you didn't offer that as a choice I will assume you can't use the balanced feed for some reason.

That said . . . Multitrap dipoles have somewhat limited bandwidth on all bands compared to a regular dipole (most obvious on 80, but also on the others). If you run high power you can cook the traps. The traps eventually fail because of weathering problems. In spite of that, they work well enough, have been used for decades and, with some work, you can build your own traps. Also you may find a design that doesn't require 135 or so feet for the total length.

Multi-dipole antennas are somewhat more difficult. They have almost the same bandwidth (for a given band) as a single dipole would have. Theory says you can build one with four dipoles and get 80-10. In practice the dipoles interact in a most unpredictable way and you will be tearing your hair out long before you get the thing to work. Settle for 3 bands and be happy to get them. You'll read or hear about people who have built these and made them work with no problems. Maybe. But they're sensitive to environment and maybe the guy got lucky. Or maybe he's exaggerating his successes.

I've built these and decided that I will settle for two bands. Beyond that the interactions and loss of bandwidth will drive you to drink. I think if you could come up with a really good way of separating the dipoles (say a 2x4 spreader at the ends) you might have better luck. Also, if you're you're looking for a DX only antenna consider a multiband vertical. The quarter wave types over a really good radial system are hard to beat. The really good radial system is the key to performance.

I've never used the half wave types, but they're said to be good. Hope that helps.

Good DX. 73, Bill/K8SYH

Third A Follow-up de Paul - K4MSG

A follow-up (with thanks to K8SYH for raising the points):

In planning a dipole, think seriously about whether or not you intend to use a transmatch, and what your rig will tolerate in terms of VSWR if you don't. My trusty old IC-735 will tolerate 2:1, but I have had it shut down before when running 30+ WPM at that VSWR (and it requires a short cool down and power recycling to bring it back up).

So, my rule of thumb is nothing higher than 1.5 or I use the transmatch. A dipole with any kind of trap will exhibit a narrower bandwidth that a plain wire. Traps can deteriorate if left out for years and never cared for, but I still have my ReyCo traps from 1962 and they have been used many times over the years with decent results. I have repainted them twice and cleaned the hardware.

In fact, I still have the trap dipole I built with them for LARG several years ago for FD, and if you want to try it you're welcome to it - in fact, I'll give it to you. Length is 132', center insulator has a SO-239 connector on it.

The W9INN dipole I use now, tuned for 1.825, 3.515, and 7.025, has the following bandwidths between VSWR=1.5 points: 160m: 12 kHz 80m: low end to 3.530 40m: low end to 7.065 Don't forget that the minimum-VSWR bandwidth of a multiband antenna can be optimized by adjusting the (coax) feedline length - a point which is fairly critical with an antenna like a G5RV, not as critical with other designs (such as a trapped or loaded dipole).

I heartily second Bill's comments about a multi-dipole with one feed. He was more blunt than I, but he's absolutely right. It boils down to how much time you want to spend out in the yard tuning and pruning and retuning and repruning, etc.

I once got a two-bander working fairly well, but when I tried to add band three I just about went insane. Again, if you can handle ladder line you only need about a 65' dipole to work 80 thru 10 with a transmatch. Lew McCoy, W1ICP, used one for decades and swore by it.

73, Paul, K4MSG

Fourth de Gary - NC4S

They both will work but usually the trap dipole will not have as great a two to one swr bandwidth as the multi dipole will. In addition you will have rf loss in the traps and the traps take more time and cost more than just simple wire antennas, if you are making the antenna yourself. If you by a commercially made antenna it will save you time.

73 Gary NC4S

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