There’s one UK company whose returns tower above all others on the London Stock Exchange over the past five years. It’s Filtronic (LSE:FTC), the AIM-listed share that’s up by roughly 4,500% in this period.

To put this mind-boggling number into context, it means £1,000 invested in May 2021 would now be worth approximately £46,000. That’s the sort of return long-term investors salivate over!

Unfortunately, penny stock Filtronic wasn’t on my radar back in 2021. But I did highlight it as an under-the-radar share to consider in March 2025 when it was near 100p (it’s now 405p). It has been regularly trumpeted on these pages.

But is it still worth a look today after skyrocketing 190% in six months?

Game-changing strategic partnership

As a quick reminder, Filtronic is a radio frequency expert. Two years ago, it signed a game-changing partnership and commercial agreement with SpaceX.

This was for the ongoing supply of its E-band solid state power amplifiers for SpaceX’s Starlink platform, as well as the development of similar products at other frequency bands.

The fact that this was a long-term strategic collaboration is what really got investors excited (the stock immediately jumped almost 50% on this news). Bumper contracts soon followed, including last year’s record-breaking £47.3m deal for Filtronic’s next-generation Gallium Nitride (GaN) powered E-band technology.

SpaceX’s Starlink provides high-speed internet to users all around the world (remote locations, cruise ships, airlines, in Ukraine, etc). Given the high likelihood that this already massive satellite constellation will grow even larger in future, the company looks perfectly placed for strong growth.

Is the stock now overvalued?

The question now is, does this all look priced in? Well, analysts currently expect revenue of £61.5m for the forthcoming fiscal year (starting June). That puts the stock on a forward-looking price-to-sales (P/S) ratio of 14.5.

For the following year, the revenue forecast stands at £75m, implying top-line growth of around 22%. But when the forward P/S ratio is above 10, I would personally be looking for stronger revenue growth than this.

Meanwhile, the forward price-to-earnings (P/E) multiple is now around 100. That makes it a very pricey stock, at least relative to near-term earnings.

Another issue worth mentioning is that a couple of company insiders have been offloading shares in recent weeks. That isn’t necessarily a red flag, but it’s noticeable that there has been no buying among directors for over a year.

What about the future?

Of course, a couple of large contracts could quickly change the picture here. And with SpaceX soon getting a massive cash injection, it will probably aggressively expand its growth plans for Starlink. That’s good news for Filtronic.

Additionally, there should be plenty of defence opportunities in Europe moving forward. Filtronic already has established relationships in this booming sector, and in March signed an $8m deal with an unnamed US firm to provide satellite communications technology.

However, while I’m still bullish on the long-term growth story (and think the stock may well move back towards 500p as SpaceX IPO excitement mounts), the valuation does concern me now.

If I had a position, I’d consider taking some profits off the table. In my experience, it’s always better to be safe than sorry when a stock goes vertical in the space of just six months.

Should you invest £5,000 in Filtronic Plc right now?

When investing expert Mark Rogers and his team have a stock tip, it can pay to listen. After all, the flagship Twelfth Magpie Share Advisor newsletter he has run for nearly a decade has provided thousands of paying members with top stock recommendations from the UK and US markets.

And right now, Mark thinks there are 6 standout stocks that investors should consider buying. Want to see if Filtronic Plc made the list?

Ben McPoland has no position in any of the companies mentioned.