Very late getting into the 10m chasing game. But better late than never, eh?
So I spent 3/4 of yesterday trying to understand the very unusual frequency response of the 5/8 wave 10m vertical I had built myself. Whilst there was a slight SWR dip around the expected frequency it was only slight, and the antenna seemed to be forever trying to tune itself for somewhere south of 28MHz. Length adjustments could remove the SWR dips but never get them into the 10m band. Tried adjusting the in-series coil away from it’s predicted 10 turns on a 25mm pipe. Again - could completely ruin the antenna but never tune the flippin’ thing!
In the end I gave up, and made myself a 10m vertical dipole - deciding I must have fundamentally misunderstood 5/8 wave verticals. Tuned fine on my deck, horizontally.
Get it up on the gable end … and again, same frequency response as the 5/8 wave.
Wasted another hour or two up and down onto the roof and up the gable-end ladder adjusting the two legs. But SWR minimum always seemed to be somewhere below 28MHz. Maybe poor connectivity between the segments due to oxide? (it’s a recycled CB antenna I scored from an old lady in town) … clutching at straws by this point.
Gave up and decided to tune up the remaining 5/8 bottle of grouse instead then went to bed.
OK. This morning. Decided to start from scratch with brand new shiny aluminium tubing. (If I’d realised how cheap the stuff was I’d have bought new to start with!). Shiny new dipole on the gable end. Same frequency response.
Hair was becoming limited, and the grouse not exactly helping.
Think back. Why did it tune OK on the deck and not on the gable end. Different coax? Fewer patch leads? Different radio? Attach dummy load and work back from the antenna feed point, as I should have done 2 days ago.
I’ve just spent 2 days testing the frequency response of my antenna switch which, it turns out, does not much like anything over about 27MHz.
Looks good for portable. But would need a non-conductive mast to use it at home.
I suppose I could sacrifice an old sota-pole if I can find a broken one that’s still good for 5m length. Would certainly give me an additional 2.5m of elevation compared to the current up-and-down dipole. And would bring the ground-half of the dipole above the roofline - which would be beneficial for both signal and noise.
Not sure how the SOTApole would like being u-bolted to the gable-end barge-boards though. We’re in ‘very high’ wind loading here - so needs to be designed with 150kph wind in mind.
Hi Matt,
I am a huge fan of a 1/4 vertical with three radials used to guy the pole if needed. To make this, I just cut the wire to the right length factoring velocity and it was resonant first time. I’ve used this on a five meter and six meter mast, no problems.
Yes - did consider a 1/4 wave vertical with radials. Though for a house mount (this thread is about a chasing antenna) it’s easier to have horizontal radials or else you have to elevate the feed point even further above the roof. Do the sloping verticals give a better radiation pattern, or do you use them for the obvious convenience when deploying portable?
I wanted to have a go at building the 5/8 wave like the Sirio Tornado I had in the '80s on the CB bands. This seemed to far outperform the 1/2 wave vertical it replaced both for LOS and DX - presumably because of the low takeoff angle benefits of the 5/8 wave design. They still make them - and for amount of stuffing around I’ve ended up doing, I should have just bought one (though the shipping from the US is more than the antenna) - but where’s the fun in that!
A 5/8th is about 200Ohms and capacitive. To get rid of the capacitive reactance you need a coil to add the opposite reactance. That removes the reactance. You normally feed the antenna from a tapping on the coil to match it to 50Ohms using it as an autotransformer.
On a 1/4wave GP, 3 radials 1/4wave long, sloping at 45degs with a 1/4wave radiator is a pretty good match to 50Ohm. Altering the angle alters the impedance.
I’ll post some pictures of a 0.64 wave 10m vertical I made, just a big longer than a 0.625 (5/8) as it ha more gain / better take off.
It’s a while since I looked at them. All I remember was they were of the order of 200-jX hence the need for +jX inductance and the impedance transformer.
I think it’s 10 years since I made mine, the mind is not so good at remember the finer details now!
Mine is a 0.64wave not a 0.625wave, 0.64 is meant to be better. That could explain the difference in resistive component values. The coil I added gets rid of the -j but that leaves R at 200 which is not going to be a good match. The tapped feed on the coil is just to make it look like 50Ohm.
Since I made mine, the intarwebz are now full of assorted 5/8 and 0.64 designs and how to match them. It would have helped me years ago
I don’t use the antenna any more, it needs an 8m pole which is heavy and it involves faffing on the summit. My 10m delta loop is simpler to setup in the field and fits my lightweight 5m pole.
The feedpoint impedance of 5/8 WL vertical monopole varies depending on the radiator length of choice. A slightly longer length will bring it to (50 to 65 range) -jX and it is convenient to match with a series L, but at that length, the radiation pattern is sensitive to the ground condition. It is better to make the radiator at 0.6 WL or slightly shorter, and deal with (200-450 ohm) - jX feedpoint impedance. This requires a shunt C and series L, or a shunt L and a series C. It is outside the match space of shunt L series L (or series L shunt L) network.
I use 5/8 WL vertical monopole where the ground soil condition is very poor. I tried one above saltwater and it was inferior to 1/4 WL. (the sidelobe with a very high takeoff angle grows big fast as the radiator becomes longer than 1/2 WL and radiated power into useless directions, i.e., wasted.) If you are looking for an all round vertical monopole, which is robust performer on any ground, I recommend 3/8 WL, which comes to (200 to 300 ohm) + jX. It works well on poor soil as well as saltwater.
Another factor is radials. Radials work best when they are a bit shorter than 1/4 WL.
The Z-f plot you posted looks like the antenna is a bit long side (for best radiation pattern) but it seems to me the problem is the matching network. Nearby objects can also have strong reaction in the antenna’s near field.
@AB1WX, @MM0FMF - thanks for the clarifications. I’ll probably have another go at this at some point (I have all the bits left to make it, after all!) and as it turned out the failure had nothing to do with the antenna.
So sounds like there are advantages to the more complicated tapped feed + capacitor approach over the simple series coil in terms of allowing lengths that give better radiation patterns and are less influenced by neighbouring structures. I’d merely adjusted the length (increased it slightly, as you guessed) to hit the 50 ohm resistive point for an easy build, without really considering the implications for the radiation pattern.
@AB1WX - yes - I had choked the feed with the simple air-wound coax approach. But had not considered the effect of the conductive mast.
As with the Sirio antenna I was basing the design off, I went for a radial length of about 0.12 lambda - which is probably the most I can get away with visually and wind-loading-wise. Any specific cons to the shorter, non-resonant radial length these antennas use?
At that length, the radial is adding additional capacitive reactance to the feedpoint impedance. If you can adjust the matching network for that, it should be ok.
A simple radial that works well is a loop of about 0.3 to 0.35 wavelengths (tie both ends of the radial wire to the feedpoint ground). I would put it in the form of a triangle or whatever shape, but the antenna should be in the middle of one side. Not much is written on this subject in English but it’s a good design.
Yes - for a 5/8 wave mounted atop a mast, the mast material and connection makes a huge difference in models. Nice low angle of takeoff with non-conductive mast. Larger mid-elevation component for mounted on (but insulated from) a scaffold pole. And then finally grounded to the scaffold pole pretty much everything goes straight up!
Non conductive support
Insulated but on a conductive mast
Earthed to a conductive mast
All for 6m base elevation.
So I guess my previous success was due to this being mounted 10m up in a tree, rather than on a metal pole.
That level of variations from a 5/8 WL is very common. As I said before, the ground soil quality has a huge impact on this, not just the support structure and the feedpoint elevation. 5/8WL (I’d rather say 0.57 to 0.6 WL to emphasize better length) is generally better suited for poor ground conditions (like most mountain summits) but not a all round performer. (3/8 WL is a all round performer.)
Slightly lower takeoff angle and slightly higher gain than 1/4 WL, and that performance is less susceptible to the ground quality than 1/4 WL. 1/4WL requires good ground to perform well but the performance goes down on poor ground or poor radial technique or both.
Another part of the advantage is that, because the feedpoint impedance and the radiation resistance are much larger than 1/4 WL, the ground loss will be much less than with 1/4WL. This factor is significant in most cases, unless you operate above the saltwater (which I often do and feel the difference).
So, the advantages over 1/4 WL are better and more robust radiation pattern and lower ground loss.
Good to know for activating or portable. But if I understandrightly you are talking about a ground mounted 1/4 wave vertical.
The question was about a chaser antenna, 6m up on a permanent mast or building, and I was interested to know the advantages of the 3/8 wave you are recommending for that situation over the elevated vertical dipole I currently chase with.
